Independent metadata management for different VOs

[elichad]

Currently metadata is not VO-independent; if one VO want new metadata everyone has to get it. Update the metadata system so VOs can manage their metadata independently of each other.

[patrick-austin]

Metadata is currently being developed in the mainline (issue #3096, pr #3424). This will allow VOs to specify metadata using their own modules and the rucio.cfg file will specify whether we allow generic metadata (which is stored as .json for individual dids), fixed metadata (things that are currently hardcoded as columns for the did table) and generic metadata with fixed keys (in which the admin will have to approve the keys for the metadata).
This seems like it may naturally lend itself to managing the metadata of different VOs, so this should be on hold until the mainline development is finished. The proposed changes are far reaching (all of the metadata functionality being moved to new files and directories) so trying to develop before it's done would be difficult.