MusicBrainz: supporting link attributes

The idea is to add link attributes as labels/properties to l_yyy_zzz relationships

SELECT
    link.id,
    link_type.link_phrase,
    link_type.name,
    array_agg(link_attribute_type.name)
FROM link
JOIN link_type ON link.link_type = link_type.id
LEFT JOIN link_attribute ON link.id = link_attribute.link
LEFT JOIN link_attribute_type ON link_attribute_type.id = link_attribute.attribute_type
GROUP BY link.id, link_type.link_phrase, link_type.name;