[Embedding models] Use indexing_embedding_versions from collection

Implement the indexing changes for using indexing_embedding_versions to generate and set embeddings according to the figjam board.

A collection can have MODELS defined:

MODELS = { 0: { field:, model: } }

A collection record should include indexing_embedding_versions which is an array of ints (default to nothing).

During indexing, embeddings are generated for every indexing_embedding_versions and stored on the corresponding field.

• Allow adding MODELS on a collection class
• Add a migration type to set indexing_embedding_versions. It should take an array of ints.
• Add a migration type to set search_embedding_version. It should take an int.
• Add documentation for how to set the initial embedding model
• Add documentation for how to switch to a new embedding model

Add migration to backfill data (#523204) is for the data backfill migrations and is out of scope of this issue.