[Libraries] [Collections] [Backend] Add tags to collections

[bradenmacdonald]

Update the edx-platform tagging/content_library APIs so that users with appropriate permissions can add/remove tags from collections. No changes to openedx-learning are expected for this ticket.

Notes:

• We can use the existing tagging mechanism, i.e. create an ObjectTag instance where object_id would be the key of the Collection being tagged
  • However, to do that we need to extend object_id field to support more keys, currently it only supports CourseKey, LibraryLocatorV2 or a UsageKey.
• We likely need to create a new type of key, CollectionKey in the opaquekeys repo, that could look something like: lib-collection:org:lib:collection-id.
  The Collection's org can be pulled from the associated ContentLibrary.

[pomegranited]

Part 2 of this ticket:

• Add convenience method that generates a Collection's object_key . Should probably live in content_libraries.api, since it needs the ContentLibrary's key?
• Build on [Libraries] [Collections] [Backend] store collections in Studio Library Index #229 to add the object_key and tags field to the collection's Meilisearch index, using the same format as used for tagged components.