add support for critique agentic search engine api for search arena.

[parthh01]

Why are these changes needed?

Critique Labs AI has developed an agentic search engine that we'd like to add to the search arena. This PR is the implementation of the instructions in FastChat/docs/model_support.md for the Critique Agentic Search Engine API.

more info on the search engine is available at @ https://critique-labs.ai

the required api-endpoints.json for running the 'model':

{
  "critique-agentic-search-api": {
    "model_name": "critique-agentic-search",
    "api_type": "critique-labs-ai",
    "api_base": "wss://api.critique-labs.ai/v1/ws/search",
    "api_key": "", 
    "anony_only": false,
    "text-arena": true,
    "vision-arena": false,
    "search-arena":true
  }
}

if the api_key is not provided, the added critique_api_stream_iter method defaults to looking for it in the CRITIQUE_API_KEY env variable.

Kindly let me know the process for providing the information needed for enabling this on the hosted lmarena. If a financial contribution would help enable this we are happy to do so.

Checks

• I've run format.sh to lint the changes in this PR.
• I've included any doc changes needed. [model registry updated]
• I've made sure the relevant tests are passing (if applicable). [haven't seen any documentation indicating tests are applicable]