Adding peeking to synapse (MSC2444 and MSC2753 support)

[ShadowJonathan]

I'd like for support for MSC2444 (Peeking over federation) and MSC2753 (Peeking via sync v2) to be added to synapse, and i'd also like for initialSync support to be removed, via which Element currently emulates room previews.

Work items;

• Implement MSC2444
  • Add in initial functionality
  • Add tests
  • Finish MSC feedback cycle and finalise code
• Implement MSC2753
  • Add in initial functionality
  • Add tests
  • Advertise to client implementations
  • Finish MSC feedback cycle and finalise code
• Fully deprecate /initialSync
  • Unblock Initial /sync when filtered to a single room can be very slow #10842
  • Issue deprecation notice
  • Remove code

[reivilibre]

#10842 is a bit of a blocker on removing /initialSync (which is already deprecated). I think some tools still have to rely on it for some uses where /sync is not optimised :/.