Allow POSTing of multiple Adjuncts in one request for creation

[p-kaczynski]

What does this change?

Impl #1110

• POST to /adjuncts now can take:
  • Adjunct object (as before)
  • JSON Array of Adjunct objects
  • HydraCollection object with members being JSON Array of Adjunct objects
• CreateOrUpdateAdjunct now assumes working on n Adjuncts by default
• Returns Adjunct, if one Adjunct was posted, or a HydraCollection of Adjuncts if multiple (TBD: currently under discussion. I believe it to be more in-line with the polymorphic input handling, and it doesn't break existing tests.)
• Tries to fail-early and saves adjuncts to the DB in a single transaction
• Returns success only if all actions (db save, ingest notify, modification notify) succeed, otherwise returns descriptive error

--

Commits:

• Initial work on multiple adjunct posting
• Rework polymorphic ItemArrayOrHydraCollectionConverter to correctly handle various input cases
• Add positive tests for multiple adjuncts posting
• Cleanup and fixes for mult adjuncts
• Add negative tests for multiple adjuncts posting

[JackLewis-digirati]

Additionally, the name of the PR could be more helpful as it goes into the commit history + the description of the PR is missing the Resolves to link the issue