Add support Vertex AI Gemini uses a public image URL (#14949)

## What

Since `langchain_google_genai.ChatGoogleGenerativeAI` supported A public
image URL, we add to support it in `langchain.chat_models.ChatVertexAI`
as well.

### Example

```py
from langchain.chat_models.vertexai import ChatVertexAI
from langchain_core.messages import HumanMessage

llm = ChatVertexAI(model_name="gemini-pro-vision")
image_message = {
    "type": "image_url",
    "image_url": {
        "url": "https://python.langchain.com/assets/images/cell-18-output-1-0c7fb8b94ff032d51bfe1880d8370104.png",
    },
}
text_message = {
    "type": "text",
    "text": "What is shown in this image?",
}
message = HumanMessage(content=[text_message, image_message])

output = llm([message])
print(output.content)
```

## Refs

-
https://python.langchain.com/docs/integrations/llms/google_vertex_ai_palm
-
https://python.langchain.com/docs/integrations/chat/google_generative_ai

libs/community/langchain_community/chat_models/vertexai.py

@@ -6,7 +6,9 @@ import logging
 import re
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any, Dict, Iterator, List, Optional, Union, cast
+from urllib.parse import urlparse
 
+import requests
 from langchain_core.callbacks import (
     AsyncCallbackManagerForLLMRun,
     CallbackManagerForLLMRun,
@@ -87,6 +89,15 @@ def _parse_chat_history(history: List[BaseMessage]) -> _ChatHistory:
     return chat_history
 
 
+def _is_url(s: str) -> bool:
+    try:
+        result = urlparse(s)
+        return all([result.scheme, result.netloc])
+    except Exception as e:
+        logger.debug(f"Unable to parse URL: {e}")
+        return False
+
+
 def _parse_chat_history_gemini(
     history: List[BaseMessage], project: Optional[str]
 ) -> List["Content"]:
@@ -118,6 +129,10 @@ def _parse_chat_history_gemini(
                         "data:image/<image_type>;base64,<base64_encoded_image>."
                     )
                 image = Image.from_bytes(base64.b64decode(encoded))
+            elif _is_url(path):
+                response = requests.get(path)
+                response.raise_for_status()
+                image = Image.from_bytes(response.content)
             else:
                 image = Image.load_from_file(path)
         else: