Search 6 groundwork - Restore support for @IndexedEmbedded.includeEmbeddedObjectId

See org.hibernate.search.annotations.IndexedEmbedded#includeEmbeddedObjectId

We have several options here:

Restore the feature as-is

It means that whenever includeEmbeddedObjectId = true and the embedded object has an @Id or @DocumentId, we will create a field of type string with the name of that id.

Pros: simple enough to implement.

Cons:

• Not very flexible (can't change the name or type of the ID field)

• Very low added value (adding @GenericField on the @Id property would achieve the same effect).

Look at the big picture

We could take advantage of an ID field in more features:

1. "id" predicate on the embedded ID: .id().field("myEmbedded").matchingAny(someCollectionOfIds), translated to a match predicate on the embedded ID field.

2. match predicate on the object field: .match().field("myEmbedded").matching(someManagedEntity), translated to a match predicate on the embedded ID field.

3. projection to entity references to the embedded entity (by passing the entity name to the backend during mapping, and using that + the projected ID when projecting)

4. projection to loaded instances of the embedded entity (using the reference and the loader passed by the mapper)

Pros:

• more consistency (same predicate for the root ID and embedded IDs)

• more features (especially projections)

Cons:

• more complex to implement, and thus will take more time.

• according to the principle of least surprise, we should enable this by default, so that predicates/projections don't fail unexpectedly.
  This means an ID field will appear in @IndexedEmbeddeds. For Lucene we can give it some internal name and it won't affect people; for Elasticsearch it's not so easy as it will be very visible in the schema and may conflict with user-defined fields... we should probably provide a way to control how that field is named? "hsearch_id" vs. "id" vs. "whatever the ID property is named" vs. "some custom name"? Or maybe just an attribute in @DocumentId: @DocumentId(fieldNameWhenEmbedded = "...")?