Add MB release annotation field as %_releaseannotation% variable

As per the thread here: https://community.metabrainz.org/t/putting-musicbrainz-annotation-into-track-comment/520489/2

It would be nice if the annotation was available as a variable for tagger scripts. It sounds like it's already available in the MB api response, so I'm hoping the code to support it in Picard isn't too much work to add.