Support Federated Entities

[luxbe]

As a user, I want to be able to load different entities from different repositories or event databases, based on the concept of Federated Queries.

An initial rough idea would be to introduce a service parameter within the @OWLObjectProperty annotation. JOPA could use this information to resolve the proper service to load the requested Entites.

@OWLObjectProperty(iri = "...", service = "abc")
private Set<Course> courses;

One central requirement for this feature is #290 - the Support for the SERVICE keyword within SOQL