REQ: add support for Contact Key Verification

[danielraffel]

iMessage's forthcoming Contact Key Verification feature is available in iOS 17.2 developer builds. It's designed to verify you're messaging with the person you intend in order to prevent sophisticated MITM attacks. It's similar in concept to existing features available on other platforms.

Service	Contact Verification Feature
WhatsApp	Security code
Signal	Safety number
Telegram	Contact key
Wickr Me	Safety code

[pixlwave]

Matrix already has a concept of this with verification - the SDK already implements it. You can see this in action in Element by tapping on a user's avatar in the timeline and then tapping Verify on the screen that appears. If they then sign in on a new device and don't verify that device for themselves, you should see a red shield on the room.