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

@@ -36,7 +36,7 @@ func NewTopologyServer(
 }
 
 // AddLink adds a new link to the topology
-func (t Topology) AddLink(ctx context.Context, request *apb.AddLinkRequest) (*apb.AddLinkResponse, error) {
+func (t *Topology) AddLink(ctx context.Context, request *apb.AddLinkRequest) (*apb.AddLinkResponse, error) {
 	sourceNode, sourcePort, err := t.ensureNodeAndPortExists(request.Link.SourceNode, request.Link.SourcePort)
 	if err != nil {
 		return nil, status.Errorf(codes.Aborted, "%v", err)
@@ -67,7 +67,7 @@ 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) {
+func (t *Topology) GetTopology(ctx context.Context, request *apb.GetTopologyRequest) (*apb.GetTopologyResponse, error) {
 	topo, err := t.topologyService.GetAll()
 	if err != nil {
 		return nil, status.Errorf(codes.Aborted, "%v", err)
@@ -103,7 +103,7 @@ func (t Topology) GetTopology(ctx context.Context, request *apb.GetTopologyReque
 	}, nil
 }
 
-func (t Topology) ensureNodeAndPortExists(incomingNode *apb.Node, incomingPort *apb.Port) (nodes.Node, ports.Port, error) {
+func (t *Topology) ensureNodeAndPortExists(incomingNode *apb.Node, incomingPort *apb.Port) (nodes.Node, ports.Port, error) {
 	node, err := t.nodeService.EnsureExists(
 		nodes.Node{
 			ID: getExistingOrCreateNewUUIDFromString(incomingNode.Id),