[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

model/plugin_cluster_event.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package model
+
+const (
+	PluginClusterEventSendTypeReliable   = CLUSTER_SEND_RELIABLE
+	PluginClusterEventSendTypeBestEffort = CLUSTER_SEND_BEST_EFFORT
+)
+
+// PluginClusterEvent is used to allow intra-cluster plugin communication.
+type PluginClusterEvent struct {
+	// Id is the unique identifier for the event.
+	Id string
+	// Data is the event payload.
+	Data []byte
+}
+
+// PluginClusterEventSendOptions defines some properties that apply when sending
+// plugin events across a cluster.
+type PluginClusterEventSendOptions struct {
+	// SendType defines the type of communication channel used to send the event.
+	SendType string
+	// TargetId identifies the cluster node to which the event should be sent.
+	// It should match the cluster id of the receiving instance.
+	// If empty, the event gets broadcasted to all other nodes.
+	TargetId string
+}