Some releases include "pre-mastered" credits separate from mastering ones (see https://ia601901.us.archive.org/25/items/mbid-a61da1da-dba6-4610-95f8-abd404fbbbe1/mbid-a61da1da-dba6-4610-95f8-abd404fbbbe1-28094887191.jpg for an example).

We could add this as an additional attribute for the existing mastering relationships, like we have "re-" already or "co-" for producer for example.

Requested by hpwg at https://community.metabrainz.org/t/pre-option-for-mastering-relationships/640723