Contract for classes (specifically, entities and components/embeddables) that are "managed". Developers can
choose to either have their classes manually implement these interfaces or Hibernate can enhance their classes
to implement these interfaces via built-time or run-time enhancement.
The term managed here is used to describe both:
the fact that they are known to the persistence provider (this is defined by the interface itself)
its association with Session/EntityManager (this is defined by the state exposed through the interface)