Teach binding-dom-codec to keep its proxies in a separate ClassLoader

As a first step towards MDSAL-401 we can start moving DataObject (possibly) and OpaqueObject (definitely) proxies into a separate ClassLoader. Since proxies do not need to load data by themselves, they do not need to reside in the same classloader as the defining type.

The classloader needs to be defined so that it has access to both the target binding class loader as well as to the classloader of binding-dom-codec. A proof that this is really the case will be successful DataObject proxy instantiation.

A real goal here is to turn ForeignOpaqueObject into an abstract class, which is then concretized in the new classloader with the implemented interface – thus forming a baseline for proxy-less invocation.