Extend support for the internal PriorityServiceLoader

RESTEASY-3090 introduced a PriorityServiceLoader. It would be useful to be able to get a collection of the types associated with that service loader. In some cases we may want to load the types, but have RESTEasy construct them as they could require forms of injection

Another possible useful scenario would be to have the ability to construct implementations with arguments. This should be used sparingly, but it could be useful in some situations.