Reintroduce default value plugin
When fixing #1272 (closed), the CXF version has been upgraded (as well as the plugins used inside). But in order to let the tests of extended work, the default value plugin has been silently removed. The fact is that default value were updated and its behaviour differs from before. Non optional fields of type object, like data field of PoK now have a default value, but it is an empty Object. Before it was a null value by default. As result not using default value let the default value be null.
However, the default value plugin is useful for other reason. We might need to update extended accordingly.