…dis#677)

When a document has a @reference @indexed field pointing to another entity,
the metamodel now generates field accessors for the referenced entity's
@indexed and @searchable fields. This enables queries like:

  entityStream.of(RefVehicle.class)
    .filter(RefVehicle$.OWNER_NAME.eq("John"))

Changes:
- MetamodelGenerator: Add processReferencedEntityIndexableFields() to
  traverse referenced entities and generate subfield accessors
- RediSearchIndexer: Add createIndexedFieldsForReferencedEntity() to
  create search index fields for referenced entity properties

Supported annotation attributes for referenced entity fields:
- @searchable: weight, sortable, nostem, noindex, phonetic, indexMissing, indexEmpty
- @TextIndexed: weight, sortable, nostem, noindex, phonetic, indexMissing, indexEmpty
- @indexed: sortable, separator, indexMissing, indexEmpty
- @TagIndexed: separator, indexMissing, indexEmpty
- @NumericIndexed: sortable, noindex, indexMissing
- @indexed (Boolean): sortable, indexMissing, indexEmpty
- @indexed (Enum): sortable, separator, indexMissing, indexEmpty

For example, given:
  @document class Owner {
    @searchable String name;
    @indexed String email;
    @TagIndexed String category;
    @NumericIndexed Integer age;
    @indexed Boolean active;
  }

  @document class RefVehicle {
    @reference @indexed Owner owner;
  }

The RefVehicle$ metamodel now includes:
  - OWNER_NAME (TextField)
  - OWNER_EMAIL (TextTagField)
  - OWNER_CATEGORY (TextTagField)
  - OWNER_AGE (NumericField)
  - OWNER_ACTIVE (TextTagField)

Note: @reference fields store only the entity ID, not the full embedded
object. To actually search by referenced entity properties, the data
must be denormalized or the query should use ReferenceField.eq(entity).

Closes redis#677