MM-21898 - Part 1: Generate and use an interface instead of *A… (#13840)

* Generate and use an interface instead of *App

app/plugin_event.go

@@ -9,8 +9,8 @@ import (
 
 // notifyClusterPluginEvent publishes `event` to other clusters.
 func (a *App) notifyClusterPluginEvent(event string, data model.PluginEventData) {
-	if a.Cluster != nil {
-		a.Cluster.SendClusterMessage(&model.ClusterMessage{
+	if a.Cluster() != nil {
+		a.Cluster().SendClusterMessage(&model.ClusterMessage{
 			Event:            event,
 			SendType:         model.CLUSTER_SEND_RELIABLE,
 			WaitForAllToSend: true,