Add function calling support

[sebastienblanc]

Is your feature request related to a problem? Please describe

Function calling is getting more and more popular and open up to some crazy use cases.

Describe the solution you'd like

In models that support function calling, the openAI API "wrapper" should return the correct json structure containing the "tools" response.

Describe alternatives you've considered

No response

Additional context

I know you've put that on the roadmap on the readme but I didn't find any issue to upvote it, so here it is :) Awesome extension btw !