core: add GRL_METADATA_KEY_MB_RECORDING_ID

MusicBrainz recording identifier. From its documentation:
A recording is an entity in MusicBrainz which can be linked to tracks
on releases. Each track must always be associated with a single
recording, but a recording can be linked to any number of tracks.

https://bugzilla.gnome.org/show_bug.cgi?id=741230