mbsync / missing plugins: consider all metadata backends

[snejus]

This PR adds support for alternate metadata backends in the mbsync and missing plugins, allowing them to work with any metadata backend the items are tagged with instead of being limited to just MusicBrainz.

Key changes:

• Removed redundant albums_for_id and items_for_id methods.
• Updated album_for_id and track_for_id implementations to return a single album/track as their names suggest.
• Updated both plugins to use the above functions instead of MusicBrainz-specific ones.
• Updated missing plugin docs to make it clear that missing albums for artists can only be obtained for the musicbrainz metadata backend
  • This is possible with other backends but implementation was outside of this PR's scope.
  • Slightly simplified the existing implementation

The changes maintain backwards compatibility while enabling users to leverage metadata from sources like Deezer, Discogs or Bandcamp when syncing their libraries.

Note: this change came about because I'm working on making musicbrainz a separate plugin: this will be submitted in the next PR.