Add http client property in serialized config

[zhangshan123]

What happened?

Describe the bug
When using company's customized model, if we do not pass the http_client parameter, the endpoint will reject our access

To Reproduce
Using the simple code:
def load_config(file_path: str = os.path.join(os.path.dirname(file), "model_config.yaml")):
with open(file_path, "r") as file:
if file_path.endswith('.json'):
config_data = json.load(file)
elif file_path.endswith('.yaml'):
config_data = yaml.safe_load(file)
if len(config_data['config'].get("api_key", "")) == 0:
config_data['config']['api_key'] = os.getenv("AZURE_API_KEY")
if config_data['config']['model'].startswith("gemini-"):
config_data['config']["http_client"] = httpx.AsyncClient(verify=False)
return config_data

model_config = load_config()
model_client = OpenAIChatCompletionClient(**model_config['config'])

Expected behavior
We can pass http_client property to AsyncOpenAI(http_client=xxx)

Screenshots
N/A

Additional context
Add http_client to BaseOpenAIClientConfiguration,such as:
class BaseOpenAIClientConfiguration(CreateArguments, total=False):
model: str
api_key: str
timeout: Union[float, None]
......
http_client: AsyncClient | None

Which packages was the bug in?

Python Extensions (autogen-ext)

AutoGen library version.

Python 0.7.5

Other library version.

No response

Model used

gemini-2.5-pro

Model provider

Azure OpenAI

Other model provider

No response

Python version

3.12

.NET version

None

Operating system

None

[zhangshan123]

I analyzed this issue but couldn't identify the affected files. Could you please provide more details about which files need to be modified?
@priteesshhh

file path:
https://github.com/microsoft/autogen/blob/main/python/packages/autogen-ext/src/autogen_ext/models/openai/config/__init__.py

#Add the http_client property to BaseOpenAIClientConfiguration, such as:
class BaseOpenAIClientConfiguration(CreateArguments, total=False):
model: str
api_key: str
timeout: Union[float, None]
max_retries: int
model_capabilities: ModelCapabilities # type: ignore
model_info: ModelInfo
add_name_prefixes: bool
"""What functionality the model supports, determined by default from model name but is overriden if value passed."""
include_name_in_message: bool
"""Whether to include the 'name' field in user message parameters. Defaults to True. Set to False for providers that don't support the 'name' field."""
default_headers: Dict[str, str] | None
http_client: AsyncClient | None