Use dataclass models in YChat

[brichet]

Fixes #115

The YChat uses dataclass models for the messages users. The User model is inherited (without change) from the jupyter server model.
It does not use dataclass for metadata, because it can contain anything. Only the id field is used by jupyterlab_chat.

This PR also save the chat file as a more readable JSON file.

EDIT: it also avoids getting all the messages in the set_message() method, where the index is already known (for performance issue on large file).

[github-actions]

👈 Launch a Binder on branch brichet/jupyter-chat/dataclass

[brichet]

Bumping CI

[brichet]

Thanks @dlqqq for the review.
This PR needs some updates, the pycrdt.Array and pycrdt.Map do not allow dataclass.
Sorry, I should have set it as draft. It should not be related to your review, though.

[brichet]

@dlqqq In this PR I changed YChat.get_messages() to "private", because it is not returning the list of Message but a list of dict, to avoid confusion.

It will collide with jupyterlab/jupyter-ai#1151, which uses this method. I can revert this change.