community[minor]: Add tidb loader support (#17788)

This pull request support loading data from TiDB database with
Langchain.

A simple usage:
```
from  langchain_community.document_loaders import TiDBLoader

CONNECTION_STRING = "mysql+pymysql://root@127.0.0.1:4000/test"

QUERY = "select id, name, description from items;"
loader = TiDBLoader(
    connection_string=CONNECTION_STRING,
    query=QUERY,
    page_content_columns=["name", "description"],
    metadata_columns=["id"],
)
documents = loader.load()
print(documents)
```

libs/community/tests/integration_tests/document_loaders/test_tidb.py

@@ -0,0 +1,76 @@
+import os
+
+import pytest
+from sqlalchemy import Column, Integer, MetaData, String, Table, create_engine
+
+from langchain_community.document_loaders import TiDBLoader
+
+try:
+    CONNECTION_STRING = os.getenv("TEST_TiDB_CONNECTION_URL", "")
+
+    if CONNECTION_STRING == "":
+        raise OSError("TEST_TiDB_URL environment variable is not set")
+
+    tidb_available = True
+except (OSError, ImportError):
+    tidb_available = False
+
+
+@pytest.mark.skipif(not tidb_available, reason="tidb is not available")
+def test_load_documents() -> None:
+    """Test loading documents from TiDB."""
+
+    # Connect to the database
+    engine = create_engine(CONNECTION_STRING)
+    metadata = MetaData()
+    table_name = "tidb_loader_intergration_test"
+
+    # Create a test table
+    test_table = Table(
+        table_name,
+        metadata,
+        Column("id", Integer, primary_key=True),
+        Column("name", String(255)),
+        Column("description", String(255)),
+    )
+    metadata.create_all(engine)
+
+    with engine.connect() as connection:
+        transaction = connection.begin()
+        try:
+            connection.execute(
+                test_table.insert(),
+                [
+                    {"name": "Item 1", "description": "Description of Item 1"},
+                    {"name": "Item 2", "description": "Description of Item 2"},
+                    {"name": "Item 3", "description": "Description of Item 3"},
+                ],
+            )
+            transaction.commit()
+        except:
+            transaction.rollback()
+            raise
+
+    loader = TiDBLoader(
+        connection_string=CONNECTION_STRING,
+        query=f"SELECT * FROM {table_name};",
+        page_content_columns=["name", "description"],
+        metadata_columns=["id"],
+    )
+    documents = loader.load()
+    test_table.drop(bind=engine)
+
+    # check
+    assert len(documents) == 3
+    assert (
+        documents[0].page_content == "name: Item 1\ndescription: Description of Item 1"
+    )
+    assert documents[0].metadata == {"id": 1}
+    assert (
+        documents[1].page_content == "name: Item 2\ndescription: Description of Item 2"
+    )
+    assert documents[1].metadata == {"id": 2}
+    assert (
+        documents[2].page_content == "name: Item 3\ndescription: Description of Item 3"
+    )
+    assert documents[2].metadata == {"id": 3}