Access MusicBrainz series (releases, release groups, recordings)

See https://musicbrainz.org/doc/Series

It would be great if beets could pull series information so you could sort e.g. podcasts like https://musicbrainz.org/series/f158ad31-bc44-4b22-94a4-025a8d6375cf into their own folder, using the ordering number in the filename, also releases/release groups that are part of a series could be sorted into some folder structure using the series or something. Either way, it would great to at least have the information handy. ;)

One caveat:
- Entities can be part of multiple series, so some way to select/prioritise between them will be needed.

Read the linked MusicBrainz Series documentation first, then trace beets' existing MusicBrainz metadata flow and its folder and filename organization behavior. Define how series data should be exposed, how multiple series are selected or prioritized, and whether ordering affects paths or filenames; the issue provides no named files or tests, so completion criteria need maintainer agreement.

Written by the indexing model from the issue text.