Add base_url configuration for anthropic claude.

[yangyu]

Is your feature request related to a problem? Please describe.
Currently in the settings, there is openai base_url can be setup but for Anthropic Claude, there is no such parameter.

Describe the solution you'd like
Just add a parameter for Anthropic Claude like anthropic_base_url and change the llm.py and other code in backend to accept it.

Describe alternatives you've considered
None

Additional context