Add option to limit key queries to users sharing rooms as per MSC4263

[Johennes]

This implements matrix-org/matrix-spec-proposals#4263.

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[CLAassistant]

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[sandhose]

Looks good to me, although this looks like it would prevent people from DMing other people who they don't share a room with? But it looks like this concern was already raised on the MSCs, so I'll leave the discussion there