Support MSC3757: Restricting who can overwrite a state event

[AndrewFerr]

Link to the MSC

Pull Request Checklist

• [x] Pull request is based on the develop branch
• [x] Pull request includes a changelog file. The entry should:
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• [x] Code style is correct (run the linters)

[AndrewFerr]

The new & edited unit tests cover the MSC's auth rules. Since they all pass, this PR is ready for review.

[clokep]

Can you expand the title of this PR? Each time I see a notification for it I wonder what MSC3757 is.

[AndrewFerr]

I think the feature will need to be opt-in via configuration rather than enabled by default.

This is already opt-in by virtue of working only in rooms of a compatible version (org.matrix.msc3757.9 / .10 / .11).

[AndrewFerr]

Also note that this has Complement tests: https://github.com/matrix-org/complement/pull/733

[AndrewFerr]

(Complement CI is expected to fail until https://github.com/matrix-org/complement/pull/733 is merged.)

[sandhose]

Changed the base branch to release-v1.116 so that I can do another RC with it included