Create JPA service callback for invoking database migration

�Looking for a simple callback/listener mechanism so that we can invoke a database migration script before JPA persistent unit starts up and runs the Hibernate schema validation mechanism.

Ideally, there would be a property within the persistence.xml that indicates which class to call. It would also pass in/inject the datasource that is being used for the persistence unit.

Essentially, this would solve the lack of migration support in Hibernate itself.