Support interface projections on reference/association properties.

Currently it is only possible to include the target type within an interface projection when operating upon associations. It should be allowed to use interface projections on references using @DBRef.

public class WithRefs { // source domain type
	@Id String id;
	@DBRef Planet planet;
}

public interface ProjectPlanetUsingOriginalTargetType {
	Planet getPlanet(); 
}

template.query(WithRefs.class).as(ProjectPlanetUsingOriginalTargetType.class) // works

public interface ProjectPlanetUsingInterfaceProjection {
	PlanetProjection getPlanet(); // fails
}

public interface PlanetProjection {
	String getName();
}

template.query(WithRefs.class).as(ProjectPlanetUsingInterfaceProjection.class) // fails

Related Issue: #3894

The issue names the template.query(...).as(...) entry point but no implementation file or test. Reproduce the failing nested @DBRef interface-projection example, trace the projection handling for references, and finish when PlanetProjection.getName() works while the existing original-target projection remains supported.