Plug-in Service Interface Registry

This blueprint is about developing a service interface registry to administer service interfaces, i.e., interfaces into the server published externally. We aim at the following goals:

1) If a service interface changes, existing plug-ins can still find correct
   versions of the interface. This mean that an old interface version, or
   a substitute for the old interface, needs to be available in the server.

2) If code changes are made within the server that do not affect the ability
   to deliver certain services, access to the services is still provided so
   that old plug-ins do not need to be re-build for the new server.

3) If new services are deployed as plug-ins, they can be registered and
   subsequently accessed using the registry.