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
```

testlib/cluster.go

@@ -20,7 +20,7 @@ func (c *FakeClusterInterface) StartInterNodeCommunication() {}
 
 func (c *FakeClusterInterface) StopInterNodeCommunication() {}
 
-func (c *FakeClusterInterface) RegisterClusterMessageHandler(event string, crm einterfaces.ClusterMessageHandler) {
+func (c *FakeClusterInterface) RegisterClusterMessageHandler(event model.ClusterEvent, crm einterfaces.ClusterMessageHandler) {
 	c.clusterMessageHandler = crm
 }