SEARCH-690 (writer): Add artist credit MBID to output

[yvanzo]

Problem: SEARCH-690 (writer part)

Search server doesn’t include MBID in results for search indexes (recording, release, release-group) that have artist credit.

Solution

• Switch mmd-schema used to write the results to an (upcoming) version that includes artist credit MBID;
• Add id attribute to artist-credit XML element.
• Update XML and JSON tests accordingly

Todolist for author

• Test it locally with unreleased mmd-schema version; See SEARCH-690 (indexer): Add artist credit MBID in indexes sir#157
• Have a new release of mmd-schema
• Use that release instead of the unreleased version in git submodule
• Test locally with that release
• Update the MusicBrainz API Search examples in XML and JSON

Action after merge

1. Release and deploy
2. After rebuilding indexes with SEARCH-690 (indexer): Add artist credit MBID in indexes sir#157,
   make the updated WikiDocs page visible on MusicBrainz website