Add basic application framework and example application to show interaction between events an NBI

Depends on:

• !340 (merged)
• !344 (merged)
• !349 (merged)
• !342 (merged)

controller/northbound/server/topology.go

@@ -47,7 +47,7 @@ func (t Topology) AddLink(ctx context.Context, request *apb.AddLinkRequest) (*ap
 		TargetNode: targetNode,
 		TargetPort: targetPort,
 	}
-	err = topologyService.AddLink(link)
+	err = t.topologyService.AddLink(link)
 	if err != nil {
 		return nil, status.Errorf(codes.Aborted, "%v", err)
 	}
@@ -60,7 +60,7 @@ func (t Topology) AddLink(ctx context.Context, request *apb.AddLinkRequest) (*ap
 
 // GetTopology returns the current topology in the form of all links
 func (t Topology) GetTopology(ctx context.Context, request *apb.GetTopologyRequest) (*apb.GetTopologyResponse, error) {
-	topo, err := topologyService.GetAll()
+	topo, err := t.topologyService.GetAll()
 	if err != nil {
 		return nil, status.Errorf(codes.Aborted, "%v", err)
 	}