Add ability to pass in classloader for ProxyFactory to use when generating proxies

When using Resteasy in OSGI environment, the classloader that loads the jax-rs annotated interfaces used in client proxy generation may not be able to see the ResteasyClientProxy class that is added by the ProxyFactory.

Proposed fix is to add ability for ProxyFactory to take a supplied classloader so user can pass in a classloader with enough context to see both the interface and the resteasy classes.