iTunes as metadata source

### Feature

iTunes plugin for tags.
You can use iTunes for album art right now, but what would be great is if you could use iTunes for other tags (e.g. ITUNESADVISORY, genre, etc.) that might not be on MusicBrainz or Discogs.

Someone has done this for Mp3Tag [(here)](https://github.com/jonaaa20/itunes-web-sources) so that might be a helpful starting point for anyone wanting to make this.

Start by examining the existing iTunes album-art integration and the iTunes web sources project linked in the issue. Determine how metadata such as ITUNESADVISORY and genre should be obtained and mapped, then verify that the plugin supplies tags unavailable from MusicBrainz or Discogs.

Written by the indexing model from the issue text.