[Libraries] [Collections] [Backend] store component-collection association in Studio Library Index

[bradenmacdonald]

This step implements the different moving parts of updating Collections data to Library Component documents in Meilisearch. To do that we need to:

• Extend the search Fields class to include collections
• Since there can be multiple collections, that field would be an array
• Add an upsert_block_collections_index_docs function updates the collections data in documents for the given library block (similar to upsert_block_tags_index_docs)
• Rename existing event CONTENT_OBJECT_TAGS_CHANGED to CONTENT_OBJECT_ASSOCIATIONS_CHANGED and add a data field to indicate which associations have changed (tags or collections). This event fires when the library component/object’s tags or collections change, and hence would call the new upsert function to update the tags and/or collections in the documents in meilisearch

Added to this issue:

• Trigger collection (re)indexing for new and updated collections, cf https://github.com/openedx/modular-learning/issues/229 + events created for https://github.com/openedx/modular-learning/issues/226

[pomegranited]

Backend changes have been deployed to the sandbox; nothing to AC test here :)