Implement `ACTION_MEDIA` controls on `MPEG_scene_interactivity` triggers in the unity player

[nlsdvl]

The initial contribution on interactivity didn't cover ACTION_MEDIA.

A test scene providing a 4 buttons to play/pause/resume/stop media should be implemented and added to the test content repository.

related:

• https://github.com/5G-MAG/rt-xr-unity-player/issues/12