Add experimental MSC3604-opt1 room version

[turt2live]

Just need a place to store this.

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

Pull Request Checklist

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[clokep]

Is this different from the org.matrix.msc2176 room version that is already implemented? It looks like it builds on v9 instead of v6? Could we just update the other one (or remove it?) Adding separate ones for each potential v10 MSC seems tedious.

[turt2live]

It builds off of v9, yea. Changing the existing one would mean changing how the room version operates, which is unsafe for rooms which might already be using it.

This is a draft and unlikely to end up in review. It's a place to point to in order to collect specific feedback and organize testing at the moment, not a candidate for inclusion in the actual project. This might change in the future though.

[turt2live]

(closing as not viable for MSC purposes)