As HibernateJpaConfiguration is package private, it is no longer
possible to extend the default Spring Boot configuration to apply
advanced settings. The most notable use case for this is the
customization of Hibernate properties using instance value vs. string
value that can be set using the "spring.jpa.properties" namespace.

This commit adds a HibernatePropertiesCustomizer callback interface that
can be implemented to tune Hibernate properties at will.

Closes gh-11211