Split the protobuf definitions out of xrootd-ssi-protobuf-interface into separate submodule

With the grpc frontend coming around, it makes sense to split the xrootd code from the actual protobuf definitions in the xrootd-ssi-protobuf-interface submodule.

We could call it cta-frontend-grpc-proto. It should be coordinated with dCache as well, as they can then migrate to this new submodule.