Option to change base_url for Google Generative AI

[Lambdua]

#783 This closed issue did not really solve the problem.

This api_endpoint can only modify like aaa.bdd.com but cannot replace the full URL address; so it's not very useful and cannot meet the requirements.

It is still necessary to add something similar to base_url modification; In fact, the gemini api already supports a similar approach:

client = genai.Client(api_key=api_key, http_options=types.HttpOptions(api_version='v1beta', base_url=base_url))

[emhagman]

#1077 will fix this if people can help and get it merged, I added support for base_url since it switches to genai.Client.