feat: add Client and API v2 (#1316)

# Description
1. Provide /api/v2 for DB-GPT
2.  Add DBGPT Python Client for Chat, Flow, App, Knowledge, Including:
   - Chat
   - Create
   - Update
   - Delete
   - Get
   - List
 3. Add examples in `examples/client/`
 4. Add API Reference document

# How Has This Been Tested?
## Test Chat Normal
### Curl
1. set `API_KEYS=dbgpt` in `.env`
2. `python dbgpt/app/dbgpt_server.py`
3. test with curl 
```
DBGPT_API_KEY=dbgpt
curl -X POST "http://localhost:5000/api/v2/chat/completions" \
    -H "Authorization: Bearer $DBGPT_API_KEY" \
    -H "accept: application/json" \
    -H "Content-Type: application/json" \
    -d "{\"messages\":\"Hello\",\"model\":\"chatgpt_proxyllm\"}"
```

```
data: {"id": "chatcmpl-ab5fd180-e699-11ee-8388-acde48001122", "model": "chatgpt_proxyllm", "choices": [{"index": 0, "delta": {"role": "assistant", "content": "Hello"}}]}

data: {"id": "chatcmpl-ab5fd180-e699-11ee-8388-acde48001122", "model": "chatgpt_proxyllm", "choices": [{"index": 0, "delta": {"role": "assistant", "content": "!"}}]}

data: {"id": "chatcmpl-ab5fd180-e699-11ee-8388-acde48001122", "model": "chatgpt_proxyllm", "choices": [{"index": 0, "delta": {"role": "assistant", "content": " How"}}]}

data: {"id": "chatcmpl-ab5fd180-e699-11ee-8388-acde48001122", "model": "chatgpt_proxyllm", "choices": [{"index": 0, "delta": {"role": "assistant", "content": " can"}}]}

data: {"id": "chatcmpl-ab5fd180-e699-11ee-8388-acde48001122", "model": "chatgpt_proxyllm", "choices": [{"index": 0, "delta": {"role": "assistant", "content": " I"}}]}

data: {"id": "chatcmpl-ab5fd180-e699-11ee-8388-acde48001122", "model": "chatgpt_proxyllm", "choices": [{"index": 0, "delta": {"role": "assistant", "content": " assist"}}]}

data: {"id": "chatcmpl-ab5fd180-e699-11ee-8388-acde48001122", "model": "chatgpt_proxyllm", "choices": [{"index": 0, "delta": {"role": "assistant", "content": " you"}}]}

data: {"id": "chatcmpl-ab5fd180-e699-11ee-8388-acde48001122", "model": "chatgpt_proxyllm", "choices": [{"index": 0, "delta": {"role": "assistant", "content": " today"}}]}

data: {"id": "chatcmpl-ab5fd180-e699-11ee-8388-acde48001122", "model": "chatgpt_proxyllm", "choices": [{"index": 0, "delta": {"role": "assistant", "content": "?"}}]}

data: [DONE]
```

### Python
```python
from dbgpt.client import Client

DBGPT_API_KEY = "dbgpt"
client = Client(api_key=DBGPT_API_KEY)
# stream
async for data in client.chat_stream(
    model="chatgpt_proxyllm",
    messages="hello",
):
   print(data)

# no stream
await client.chat(model="chatgpt_proxyllm", messages="hello")

```
## Test Chat App
### Curl
 test with curl 
```
 DBGPT_API_KEY=dbgpt
 APP_CODE={YOUR_APP_CODE}

curl -X POST "http://localhost:5000/api/v2/chat/completions" \
    -H "Authorization: Bearer $DBGPT_API_KEY" \
    -H "accept: application/json" \
    -H "Content-Type: application/json" \
    -d "{\"messages\":\"Hello\",\"model\":\"chatgpt_proxyllm\", \"chat_mode\": \"chat_app\", \"chat_param\": \"$APP_CODE\"}"

```
### Python
```python
from dbgpt.client import Client

DBGPT_API_KEY = "dbgpt"
client = Client(api_key=DBGPT_API_KEY)
APP_CODE="{YOUR_APP_CODE}"

async for data in client.chat_stream(
     model="chatgpt_proxyllm",
     messages="hello",
     chat_mode="chat_app", 
     chat_param=APP_CODE
    ):
        print(data)

```


# Snapshots:

Include snapshots for easier review.

# Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have already rebased the commits and make the commit message
conform to the project standard.
- [x] I have performed a self-review of my own code
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have made corresponding changes to the documentation
- [x] Any dependent changes have been merged and published in downstream
modules

dbgpt/core/schema/__init__.py

@@ -0,0 +1 @@
+"""Module for common schemas."""

dbgpt/core/schema/api.py

@@ -0,0 +1,116 @@
+"""API schema module."""
+
+import time
+import uuid
+from typing import Any, Generic, List, Literal, Optional, TypeVar
+
+from dbgpt._private.pydantic import BaseModel, Field
+
+T = TypeVar("T")
+
+
+class Result(BaseModel, Generic[T]):
+    """Common result entity for API response."""
+
+    success: bool = Field(
+        ..., description="Whether it is successful, True: success, False: failure"
+    )
+    err_code: str | None = Field(None, description="Error code")
+    err_msg: str | None = Field(None, description="Error message")
+    data: T | None = Field(None, description="Return data")
+
+    @staticmethod
+    def succ(data: T) -> "Result[T]":
+        """Build a successful result entity.
+
+        Args:
+            data (T): Return data
+
+        Returns:
+            Result[T]: Result entity
+        """
+        return Result(success=True, err_code=None, err_msg=None, data=data)
+
+    @staticmethod
+    def failed(msg: str, err_code: Optional[str] = "E000X") -> "Result[Any]":
+        """Build a failed result entity.
+
+        Args:
+            msg (str): Error message
+            err_code (Optional[str], optional): Error code. Defaults to "E000X".
+        """
+        return Result(success=False, err_code=err_code, err_msg=msg, data=None)
+
+
+class DeltaMessage(BaseModel):
+    """Delta message entity for chat completion response."""
+
+    role: Optional[str] = None
+    content: Optional[str] = None
+
+
+class ChatCompletionResponseStreamChoice(BaseModel):
+    """Chat completion response choice entity."""
+
+    index: int = Field(..., description="Choice index")
+    delta: DeltaMessage = Field(..., description="Delta message")
+    finish_reason: Optional[Literal["stop", "length"]] = Field(
+        None, description="Finish reason"
+    )
+
+
+class ChatCompletionStreamResponse(BaseModel):
+    """Chat completion response stream entity."""
+
+    id: str = Field(
+        default_factory=lambda: f"chatcmpl-{str(uuid.uuid1())}", description="Stream ID"
+    )
+    created: int = Field(
+        default_factory=lambda: int(time.time()), description="Created time"
+    )
+    model: str = Field(..., description="Model name")
+    choices: List[ChatCompletionResponseStreamChoice] = Field(
+        ..., description="Chat completion response choices"
+    )
+
+
+class ChatMessage(BaseModel):
+    """Chat message entity."""
+
+    role: str = Field(..., description="Role of the message")
+    content: str = Field(..., description="Content of the message")
+
+
+class UsageInfo(BaseModel):
+    """Usage info entity."""
+
+    prompt_tokens: int = Field(0, description="Prompt tokens")
+    total_tokens: int = Field(0, description="Total tokens")
+    completion_tokens: Optional[int] = Field(0, description="Completion tokens")
+
+
+class ChatCompletionResponseChoice(BaseModel):
+    """Chat completion response choice entity."""
+
+    index: int = Field(..., description="Choice index")
+    message: ChatMessage = Field(..., description="Chat message")
+    finish_reason: Optional[Literal["stop", "length"]] = Field(
+        None, description="Finish reason"
+    )
+
+
+class ChatCompletionResponse(BaseModel):
+    """Chat completion response entity."""
+
+    id: str = Field(
+        default_factory=lambda: f"chatcmpl-{str(uuid.uuid1())}", description="Stream ID"
+    )
+    object: str = "chat.completion"
+    created: int = Field(
+        default_factory=lambda: int(time.time()), description="Created time"
+    )
+    model: str = Field(..., description="Model name")
+    choices: List[ChatCompletionResponseChoice] = Field(
+        ..., description="Chat completion response choices"
+    )
+    usage: UsageInfo = Field(..., description="Usage info")