Add base_url configuration for anthropic claude.

**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**