Ringing acknowledgements

### Your use case

#### What would you like to do?
EX support for Ringing acknowledgements as per [spec](https://github.com/matrix-org/matrix-spec-proposals/blob/toger5/matrixrtc-call-ringing/proposals/4075-rtc-notification-event.md#ringing-acknowledgements)
#### Why would you like to do it?
Establish whether the callee phone is actually ringing instead of giving a fake feedback ring as we do now.
#### How would you like to achieve it?
EX should implement the spec.

### Have you considered any alternatives?

Major messaging clients now support this.

### Additional context

_No response_

### Are you willing to provide a PR?

No

Start by reading the linked Matrix spec for ringing acknowledgements and locating Element X Android's existing call-ringing flow. Determine where the current feedback ring is produced and define done as reporting the callee's actual ringing state according to the spec, with coverage for the acknowledgement behavior.

Written by the indexing model from the issue text.