Add opt-in AirPlay 2 playback relay control - like iOS Control Center

[DerPicknicker]

Hey,

I figured out how the Observer role works in AP2. Like on iOS so you're able to control your AP2 Speaker which is not paired in Apple HomeKit + not streaming from MA or from pyATV.

I've tested it locally - code is written by Codex.

Here is the fully description:
Adds an experimental MrpTunnel.Relay mode for observing and controlling existing playback through an AirPlay 2 receiver using pyatv’s standard APIs.

Supported functionality:

• Live track metadata, playback state, and position.
• Album-cover retrieval through metadata.artwork(), using existing artwork retrieval and caching.
• Play, pause, toggle, next/previous track, and seeking.
• Absolute volume control for the selected receiver or an explicitly selected group member.
• Playback commands addressed to the active sender/player, with sender rejection errors surfaced to callers.
• Volume changes confirmed by receiver updates.
  The implementation uses an authenticated, encrypted AirPlay control-only session and reuses existing message and player-state handling. Includes a discovery/control example and protocol documentation. Existing connection defaults remain unchanged.
  Validation: Live receiver testing confirmed metadata, album-cover retrieval, pause/resume, seeking, and volume. Next/previous and command failures are covered by protocol tests. Full suite: 1,271 passed, 6 skipped; lint, typing, and generated-file checks passed.
  Current scope: Transient AirPlay pairing. HomeKit authentication and broader receiver compatibility remain future work.

[DerPicknicker]

@postlund open for any feedback, I've built a debugging toolkit as well. If'youre interested, I can add you privately. DM me on Discord.

[DerPicknicker]

Artwork should now also work - tested locally - Code written by Codex.

Any chances this could be merged quickly so Music Assistant and Home Assistant can benefit from it @postlund ?

[postlund]

Artwork should now also work - tested locally - Code written by Codex.

Any chances this could be merged quickly so Music Assistant and Home Assistant can benefit from it @postlund ?

I'll try to find some time within the next few days!

[DerPicknicker]

Thank you :)