Add support for MSC1929 (/.well-known/matrix/support)

[girlbossceo]

(matrix-org/matrix-spec-proposals#1929)

Unsure how we can do this on the server-side right now, but there's a Dendrite issue for it that hasn't been rejected and if we can do client/server well-knowns from server-side, this should be possible too.

Currently I'm doing this from the reverse proxy: https://puppygock.gay/.well-known/matrix/support

[girlbossceo]

Implemented this, implemented support for m.role.moderator (MSC4121), and implemented an admin command to fetch support well-known.

Only supports 1 contact due to the amount of complexity involved in creating a config option like well_known_contacts = [["user1", "security", "@user1"], ["user2", "moderator", "@user2]]

I believe it would require implementing a custom type and a custom serde serialiser.

If a user needs more than 1 contact, they should consider just serving this file themselves.