Add album art support in the mpd component (#44527)

* Add album art support in the mpd component

Uses `readpicture` to retrieve embedded artwork and `albumart` to
acquire cover art located in the media directory.

As the mpd component supports multiple different implementations (think
mopidy, PI MusicBox, etc.) we check for the availability of each command
before using them.

Tested against mpd 0.22.3, which includes support for both.

* fixup! Add album art support in the mpd component