Ability to add labels or tags

[pgaschuetz]

I would like to be able to add labels (or "tags") to albums, songs, playlists or radio stations - no matter the provider and be able to filter by them in the UI or query them from HomeAssistant.

This would allow users to i.e. play music labelled with "Dinner", "Morning", "Lounge", "Party", etc.
Ideally, multiple labels could be assigned per item.

I understand, that I can mark items as favorites or create playlists, but this does not allow for easy "scene" based selection of tracks.

This might be related to #1559 and #2419