Take into account PartType in RESTEasy's provider detection

[gsmet]

When working with multipart, org.jboss.resteasy.annotations.providers.multipart.PartType can reference other media types and might require additional providers to be registered.

At the moment, we totally ignore it.

This is more important with JSON being the default as a lot of users didn't properly registered the Consumes/Produces and thus all the providers were registered. Now that we only register JSON by default, there might be some missing providers.