Ensure embeddable single-column identifier references and embedded prefixes are considered properly

Follow up to #2113. We should make sure that converters for custom ID types are applied correctly and that any prefixes for composite ID's are considered correctly, also for AggregateReference:

class Foo {

  @Id Long id;
  AggregateReference<Bar, BarId> assignedTo;
}

class Bar {

  @Id @Embedded.Nullable(prefix = "id_") BarId id;
}

class BarId {

  String a;
  String b;
}

and

class Foo {

  @Id Long id;
  @Embedded.Nullable(prefix = "assigned_to_") AggregateReference<Bar, BarId> assignedTo;
}

class Bar {

  @Id BarId id;
}

Start by reading the linked follow-up issue #2113 and tracing the existing converters for custom ID types, embedded prefixes, and AggregateReference handling. Use the two Java examples as regression cases; done means single-column custom IDs and composite IDs with prefixes are mapped correctly in both forms.