JSON mode for assistants

[fjsj]

It works, just need to be documented better. See draft work at #52

We need a nice example. It could be the Tour Guide (see TourGuideAssistant in tests). It's not a chat, it receives a location from the user, using the browser location. Geocode that location (get the address from the lat/lng), send to LLM, and show cards with interesting attractions around. Not sure how to filter what's a tourist attraction or not to send to the LLM, though.

Edit: this is now more important to be supported by the lib API due to strict mode in OpenAI: https://openai.com/index/introducing-structured-outputs-in-the-api/

[fjsj]

Solved by #171