feat: allow patching req body with client config

[sigoden]

Each client has a new patches configuration

 - type: <client>
   patches: 
      <regex>:                                    # The regex to match model names, e.g. '.*' 'gpt-4o' 'gpt-4o|gpt-4-.*'
        request_body:                             # The JSON to be merged with the request body.

We can use this pattern to modify the request body.

Enable web-search for cohere models

  - type: cohere
    patches:
      ".*":
        request_body:
          connectors:
            - id: web-search

Change safetySettings of gemini models

  - type: gemini
    patches:
      'gemini-.*':                                           
        request_body:                                 # Override safetySettings for gemini models
          safetySettings:
            - category: HARM_CATEGORY_HARASSMENT
              threshold: BLOCK_NONE
            - category: HARM_CATEGORY_HATE_SPEECH
              threshold: BLOCK_NONE
            - category: HARM_CATEGORY_SEXUALLY_EXPLICIT
              threshold: BLOCK_NONE
            - category: HARM_CATEGORY_DANGEROUS_CONTENT
              threshold: BLOCK_NONE

This PR replaces the previous strategy of adding fields through extra_fields #298

[jiahut]

@sigoden report bug ( patched the web-search connector )

> .model cohere:command-r-plus


> 最近一个月发布了哪些开源的LLM
Failed to get answer

Caused by:
    incomplete utf-8 byte sequence from index 2749

The above error did not occur consistently. I conducted three consecutive inquiries, and the first two attempts resulted in errors, while the third attempt successfully received a response.

[sigoden]

#538 fixed the bug