LPS-49081

Hey Brian,

This is an other set of changes for customizing the XStream library from our portlet data handlers. This time it's about XStream converters. We would need a custom converter for DL to properly handle the different repository information export/import.

This change is providing a framework for this, using the adapter pattern for the XStream converter, to meet the  modular requirements. Also it is providing a registry so the coders can register their own converters.
We needed a "two way adapter" since we wanted to provide an interface for the developers to implement, but we needed a backwards adapter to be able to "feed" the custom ones to the XStream itself.

We will send the apply in a different pull request, we wanted to push the framework first to master.

Thanks,

Máté