To implement call declines on EX we need to expose this event type to the FFI.

Most context can be found in the PR implementing this event in ruma: ruma/ruma#2198

We need:

• to send the event from Element X in the notification worker (while showing the full screen notification)
• we need to be able to listen to a room timeline when we are currently playing a call ring for this event type to stop ringing on all other devices if such an event arrives.