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
```

einterfaces/cluster.go

@@ -12,7 +12,7 @@ type ClusterMessageHandler func(msg *model.ClusterMessage)
 type ClusterInterface interface {
 	StartInterNodeCommunication()
 	StopInterNodeCommunication()
-	RegisterClusterMessageHandler(event string, crm ClusterMessageHandler)
+	RegisterClusterMessageHandler(event model.ClusterEvent, crm ClusterMessageHandler)
 	GetClusterId() string
 	IsLeader() bool
 	// HealthScore returns a number which is indicative of how well an instance is meeting