Chat: Anthropic chat client for cody.dev.models

[abeatrix]

CLOSE https://linear.app/sourcegraph/issue/CODY-2431

This PR adds a custom chat client for anthropic when an anthropic model is provided through the cody.dev.models config.

This allows us to test new models that are not yet available through Cody Gateway in the client locally.

Test plan

1. Build Cody from this branch
2. In your VS CODE settings file, configure cody.dev.models to add a new claude model. For example:

   {
   "cody.dev.models": [
       {
           "provider": "anthropic",
           "model": "claude-3-new-model-name",
           "apiKey": "$GET_ACCESS_TOKEN_FROM_1PASSWORD"
       },
   ]
   }

3. Reload VS Code
4. Verify you can choose the new model from the chat model dropdown list
5. Verify you are getting response back when using the new model