mbsync: add field filtering option

[frnzhop]

The problem with mbsync is that it overwrites all fields that differ from MusicBrainz, with no way to exclude specific ones. This creates the following:

1. Conflicts with other plugins: lyrics and replaygain store values that MusicBrainz doesn't have. Running mbsync wipes them out.
2. No selective sync: if I want to pick up an update (e.g. a corrected album title or artist credits), I have to run a full sync and accept all the side effects.

The only workaround is to integrate the MusicBrainz API manually via script, which defeats the purpose of installing this plugin in the first place.

Proposed solution

A --fields (or -f) option to whitelist or blacklist specific fields during sync. For example:

beet mbsync --fields title,artist,album       # only sync these fields
beet mbsync --exclude-fields lyrics,replaygain  # sync everything except these

This would make mbsync much more flexible for usage where not all metadata comes from MusicBrainz.

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A similar issue was mentioned in #6275 but the discussion was closed before it could be addressed.

[snejus]

What is your overwrite_null configuration?