Return an object of the specified type to allow access to the provider-specific API. If the provider's
EntityManager implementation does not support the specified class, the
PersistenceException is thrown.
cls - the class of the object to be returned. This is normally either the underlying
EntityManager implementation class or an interface that it implements.
- an instance of the specified class
PersistenceException - if the provider does not support the call
- JPA 2.0