Support for JPA 4.0 `@PersistenceAgent` injection

#36025 delivers basic `EntityAgent` support around Spring-style autowiring for a shared transactional proxy, against JPA 4.0 M1 and Hibernate 8.0.0.Alpha1.

As a follow-up, `@PersistenceAgent` injection in `PersistenceAnnotationBeanPostProcessor` can be delivered once it gets available in a later JPA 4.0 milestone.