Allow to persist entities

[omilke]

As a developer I want to supply entity-objects to be persisted, in order to provide fully initialized entities.

I have factory methods on my entities that set them up internally. In order to reuse this encapsulation I want to supply entity objects to be persisted. The same applies for entities that have Builders.

This is especially helpful when your JPA entities are more like entities in the sense of Domain Driven Design.