Improve EntityManager injection

[ivannov]

So far EntityManager producer relies that there is just one persistence unit defined in persistence.xml. However, this is not always the case. It should be possible to configure that this way:

@Inject
@PersistenceUnitName("myUnit")
private EntityManager em;

Another thing to be taken into account is the way to initialize the EntityManagerFactory. Consider this blog post: https://vladmihalcea.com/2017/12/19/how-to-bootstrap-jpa-programmatically-without-the-persistence-xml-configuration-file/