Implement sets CRUD for OAI-PMH - backend work

This is a UXPROD-2406 spillover from 2020 R3 with the backend work for implementing sets for OAI-PMH.

Currently, OAI-PMH allows selective harvesting only by date.
With sets implementation, it is going to provide an opportunity to harvest by different fields, e.g. location, record type, discovery suppression etc.

Sets configuration should be flexible enough and available via user interface, so that librarians can form their own sets structure. Separate page “Sets” in Settings->OAI-PMH is to be created. Here sets can be searched, viewed, created, duplicated, modified and deleted, according to the user’s permissions (see designs attached).

More information on sets can be found by link http://www.openarchives.org/OAI/2.0/openarchivesprotocol.htm#Set.

Use cases:
I would like to be able to combine a bunch of locations into a single set. We have an associated K-12 school with its own library as part of our structure. I would like to harvest just the K-12 library data. I would like to create a set that includes all of the K-12 locations and not a separate set for 200 locations.

Special collections libraries might want to contribute a set of their records to a union catalog. Perhaps that might be based on subject. It would definitely be done for all records of items that have digitizations (think DPLA or Europeana). Or even just to match the metadata records with their own digital assets management systems, for combined display in discovery systems, since DAMS often have much less robust descriptive metadata.

Questions:

1. Is selecting one location going to resolve the problem?
2. Can multiple sets be used during one harvest?
3. Which of the following is the correct approach for saving setSpecs:

• <setSpec>test</setSpec>
• <setSpec>mycustomentry</setSpec><setSpec>audio-role-only</setSpec>
• <setSpec>mycustomentry:audio-role-only</setSpec>