EZP-23400: ES: Implement UserMetadata::GROUP data indexing and visitors
EZP-23400: Solr: Implement UserMetadata::GROUP data indexing and visitor
EZP-23400: added tests
Merge pull request #1045 from ezsystems/impl-EZP-23400-visite-usermetadata-group-2

Implement visiting Criterion\UserMetadata::GROUP target