Add a Javassist proxy serialization classresolver

Javassist proxies can be fully serialized with a little support. A class resolver can do the job. However it should not be an integral part of the protocols because we don't want to add a dependency on javassist; in a similar vein, any maven dep needs to be declared "optional" or "provided", whichever one is right (I always forget).

Just create a class resolver and add it into the API package to be available to users.