ChatAPI Updates

[mikeburgh]

Once they have been out for a while, support the following:

Enable model selection in chat window

• https://code.visualstudio.com/updates/v1_95#_chat-participant-access-to-model-picker

Expose schema as a tool for other chat providers

• https://code.visualstudio.com/updates/v1_95#_tools-for-language-models