- Added version check
- Added 'invitee' to m.call.invite
- Added 'party_id' to all VoIP events
- Added new events
  - m.call.select_answer
  - m.call.reject
  - m.call.negotiate
- Added reason codes to m.call.hangup
- Added custom struct for session description objects

https://spec.matrix.org/v1.2/client-server-api/#token-authenticated-registration

This isn't ready to be used yet because the patch for synapse hasn't been
merged (https://github.com/matrix-org/synapse/pull/11867).