MM-21357: Use typed constants ClusterEvent (#17920)

* MM-21357: Use typed constants ClusterEvent

Use typed constants for:
- model.ClusterEvent

https://mattermost.atlassian.net/browse/MM-21357

```release-note
Use typed constant for model.ClusterEvent
```

* trigger CI
```release-note
NONE
```

app/busy_test.go

@@ -126,7 +126,7 @@ func (c *ClusterMock) SendClusterMessageToNode(nodeID string, msg *model.Cluster
 
 func (c *ClusterMock) StartInterNodeCommunication() {}
 func (c *ClusterMock) StopInterNodeCommunication()  {}
-func (c *ClusterMock) RegisterClusterMessageHandler(event string, crm einterfaces.ClusterMessageHandler) {
+func (c *ClusterMock) RegisterClusterMessageHandler(event model.ClusterEvent, crm einterfaces.ClusterMessageHandler) {
 }
 func (c *ClusterMock) GetClusterId() string                                       { return "cluster_mock" }
 func (c *ClusterMock) IsLeader() bool                                             { return false }

app/plugin_event.go

@@ -7,7 +7,7 @@ import (
 	"github.com/mattermost/mattermost-server/v5/model"
 )
 
-func (s *Server) notifyClusterPluginEvent(event string, data model.PluginEventData) {
+func (s *Server) notifyClusterPluginEvent(event model.ClusterEvent, data model.PluginEventData) {
 	if s.Cluster != nil {
 		s.Cluster.SendClusterMessage(&model.ClusterMessage{
 			Event:            event,