Bluetooth: Audio: Media proxy

This commit adds the media proxy from the topic-le-audio branch.
This is a part of the upmerge of the le-audio media control files.

The media proxy adds a common API to register and access local media
players, and to discover and access remote media players over
bluetooth using the media control client and a remote media control
service.

The commit is a pure copy of the files and content in the
topic-le-audio branch, with the following exceptions:

- files are in bluetooth/audio instead of bluetooth/host/audio, with
  some include paths updated as a consequence
- as a consequence, CMake files and Kconfig files updates are done in
  other locations

Signed-off-by: Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>