Allow customization of awareness field keys (`user` and `cursor`)

[ysds]

Is your feature request related to a problem? Please describe.
I'm developing a canvas application and trying to add BlockNote nodes to it. However, my application already uses the user and cursor awareness fields, which conflicts with BlockNote's built-in awareness fields.

Describe the solution you'd like
I would like to have configuration options to customize the awareness field keys for user and cursor in BlockNote. This would allow developers to specify custom field names (e.g., bnUser, bnCursor) to avoid conflicts with existing awareness implementations in their applications.

Describe alternatives you've considered
Restructuring my application's awareness system to use field names that don't conflict with BlockNote's built-in fields (would require refactoring of existing code)

Additional context
y-prosemirror - cursor-plugin (cursorStateField)

Bonus
[ ] I'm a sponsor and would appreciate if you could look into this sooner than later 💖

[nperez0111]

As far as I know, the cursorStateField in y-prosemirror is not working properly at the moment (with an open PR to address it) yjs/y-prosemirror#159 so we'd have to fix upstream before we can open the API for it here