MatrixRTC: Introduce key transport abstraction as prep work for to-device encryption (#4773)
* refactor: extract RoomKeyTransport class for key distribution
* refact: Call key transport, pass the target recipients to sendKey
* update IKeyTransport interface to event emitter.
* fix not subscribing to KeyTransportEvents in the EncryptionManager + cleanup
* fix one test and broken bits needed for the test (mostly statistics wrangling)
* fix tests
* add back decryptEventIfNeeded
* move and fix room transport tests
* dedupe isMyMembership
* move type declarations around to be at more reasonable places
* remove deprecated `onMembershipUpdate`
* fix imports
* only start keytransport when session is joined
* use makeKey to reduce test loc
* fix todo comment -> note comment
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Co-authored-by: Timo <toger5@hotmail.de>