I'd like to deprecate the create_chat_func in favor of a client argument which could be a function, as in create_chat_func, but could also be an ellmer::Chat object.

This is feasible because we only need to set the system prompt

which is possible using the $set_system_prompt() method.

This does introduce a few new issues:

1. If client is a Chat object, we'll call $clone() to create a new chat.
2. Should we remove turns from the client?
   • I think yes: it maintains current behavior.)
3. Should we remove additional tools from the client?
   • I think maybe not, that could be an interesting way to extend querychat.