Cosmos: support partial property loading (e.g. for vector properties)

#37277 added support for not loading vector properties at the metadata level, and tracks implementing on the relational side.

Cosmos is tricky, since we typically load the entire document as-is, either via ReadItem or via a query projection of the entire document; there's no feature currently to exclude a specific property - that might happen in the future.

For a solution today, if an entity is projected that has non-autoloaded properties, we could instead project all properties *except* for those properties; this would also mean transforming ReadItem to a query (it should be worth it).

Tentatively proposing for 11.0, though we may not get around to it.