When class is annotated with @MappedSuperclass, its private fields can't be accessed from JPA with native image

[Tomas-Kraus]

Added set of jUnit tests to demonstrate this issue and verify that it's fixed.
Fixed native image support to process @MappedSuperclass private fields properly.

[tomas-langer]

I have done a big change in HelidonReflectionFeature (switching to a scanning tool and no longer using unsupported APIs in this class).

[tomas-langer]

@ljnelson could you please review as well to make sure we use as much of the API as possible (and in the right way). Thanks

[Tomas-Kraus]

Applied this patch on top of the master branch and see tests failures with native image. All of them have the same cause:

Exception: No identifier specified for entity: io.helidon.tests.integration.jpa.model.City
  org.hibernate.cfg.InheritanceState.determineDefaultAccessType(InheritanceState.java:266)
  org.hibernate.cfg.InheritanceState.getElementsToProcess(InheritanceState.java:211)
  org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:781)
  org.hibernate.boot.model.source.internal.annotations.AnnotationMetadataSourceProcessorImpl.processEntityHierarchies(AnnotationMetadataSourceProcessorImpl.java:254)
  org.hibernate.boot.model.process.spi.MetadataBuildingProcess$1.processEntityHierarchies(MetadataBuildingProcess.java:230)

This is what google knows about this exception:
You are missing a field annotated with @Id. Each @Entity needs an @Id - this is the primary key in the database.

But City @Entity extends Settlement @MappedSuperclass which has @Id annotation defined. Such a model works fine outside native image.

[Tomas-Kraus]

Exception: No identifier specified for entity: io.helidon.tests.integration.jpa.model.City was fixed by adding MappedSuperclass into reflection config.

[tomas-langer]

LGTM