Expose ProtoBuf Manager through DMR

The ProtoBuf Manager should be exposed through the DMR as a child resource under cache-container nodes. The resource should expose a number of operations to list, get, set, remove installed schemas.

Example:
.../cache-container=container/protobuf-schemas=PROTOBUF-SCHEMAS

:list-schemas()
:get-schema(name=myschema)
:remove-schema(name=myschema)
:set-schema(name=myschema, schema-source=...)