Add user_may_create_room_with_visibility spamchecker module API callback

[hughns]

WIP

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[hughns]

It might be preferable from a module API point of view to separate room creation from changing visibility.

For changing visibility it might make sense to be:

user_may_change_visibility(user_id: str, room_id: str, old_visibility: str, new_visibility: str)

But, that might also be over complicating things.