the JtaPersistenceUnitPostProcessor enables us to define the JTA capable datasource
which should be used by the JPA provider directly as a reference within the Spring configuration
file.
enrich the PersistenceUnitInfo read from the persistence.xml configuration file with a reference
to the jtaDataSource injected via the Spring configuration. the JTA capable datasource is then
used by the LocalContainerEntityManagerFactoryBean to create the EntityManagerFactory