DATAJPA-612 - Allow @EntityGraph on a method inherited from JpaRepository.

Proof of concept fix - PLEASE do not merge!

We now respect the @EntityGraph annotations on methods that override SimpleRepository methods. CrudMethodMetadata now looks for @EntityGraph annotations, and if present the appropriate entityGraph configuration is applied within SimpleJpaRepository#applyRepositoryMethodMetadata.

Previously we mistakenly treated the @EntityGraph annotation as a Query annotation which triggered a queryResolution process that tried to find a named or create a derived query and failed.
Removed @QueryAnnotation from EntityGraph, since it should only be used to mark store specific @Query annotations.

Renamed Jpa21QueryCustomizer to Jpa21Utils.
Add `@return` with nullabilty constraints.