Store Collection metadata + component count in meilisearch

[pomegranited]

Follow-up to openedx/modular-learning#229 + #1102

on receipt of LIBRARY_COLLECTION_CREATED / LIBRARY_COLLECTION_UPDATED:

• Create/update the Collection metadata stored in Meilisearch
• Add an additional field to the Collection document: num_children, which stores collection.entities.count()

Note: we'll deal with LIBRARY_COLLECTION_DELETED in openedx/modular-learning#231