Add MSC3604 opt3 + prior

[turt2live]

This is largely for MSC implementation proof and isn't really expected to merge.

MSC: matrix-org/matrix-spec-proposals#3604

Composition of:

• Add experimental MSC3604-opt1 room version #11662
• Support MSC3667 in a new room version #11885
• Add a new room version for MSC3787's knock+restricted join rule #12623
• Additional commits as needed and to add opt3 itself

[turt2live]

PR has served its purpose.