Messaging fabric plugins (#592)

* Plugin architecture for messaging fabric

* Schemas use a technology neutral expression

* Schemas strictness has uncovered some incorrect schema use which is fixed

trustgraph-flow/trustgraph/agent/react/service.py

@@ -433,13 +433,11 @@ class Processor(AgentService):
                     end_of_dialog=True,
                     # Legacy fields for backward compatibility
                     error=error_obj,
-                    response=None,
                 )
             else:
                 # Legacy format
                 r = AgentResponse(
                     error=error_obj,
-                    response=None,
                 )
 
             await respond(r)