Add callbacks for debugging

[petrus-jvrensburg]

This PR would add two new params to chat_completion, namely inspect_request? and inspect_response? which has the effect of calling IO.inspect on the request, just before sending it to the LLM, or on the response before processing it.

I needed this for debugging, so I think others might find it useful.

@thmsmlr How does this sit with you? An alternative might be to pass in a callback, in stead of setting a flag, and then the user can log / inspect or do whatever they want in the callback. Any thought?