Draft: vlc_aout: add vlc_audio_output_callbacks for device hotplug support in player

The callbacks are similar to those from vlc_audio_output_events but are meant to forward to the interfaces/player instead of the audio output owner. They could be unified in later refactors.

The Qt interface doesn't implement the callback correctly yet, I'm waiting for approval on the way I plan to refactor the audio output events vlc_variable into the callback structure in input_resource.

Fixes #22478

Refs #22479