[MM-35077] Add basic support for plugin intra-cluster communication (#17495)

* Add basic support for plugin intra-cluster communication

* Some renaming for added clarity

* Allow sending cluster event to specific nodes

* Improve naming and documentation

* Improve logging

einterfaces/cluster.go

@@ -21,7 +21,8 @@ type ClusterInterface interface {
 	HealthScore() int
 	GetMyClusterInfo() *model.ClusterInfo
 	GetClusterInfos() []*model.ClusterInfo
-	SendClusterMessage(cluster *model.ClusterMessage)
+	SendClusterMessage(msg *model.ClusterMessage)
+	SendClusterMessageToNode(nodeID string, msg *model.ClusterMessage) error
 	NotifyMsg(buf []byte)
 	GetClusterStats() ([]*model.ClusterStats, *model.AppError)
 	GetLogs(page, perPage int) ([]string, *model.AppError)