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/mocks/ClusterInterface.go

@@ -187,7 +187,7 @@ func (_m *ClusterInterface) NotifyMsg(buf []byte) {
 }
 
 // RegisterClusterMessageHandler provides a mock function with given fields: event, crm
-func (_m *ClusterInterface) RegisterClusterMessageHandler(event string, crm einterfaces.ClusterMessageHandler) {
+func (_m *ClusterInterface) RegisterClusterMessageHandler(event model.ClusterEvent, crm einterfaces.ClusterMessageHandler) {
 	_m.Called(event, crm)
 }
 

einterfaces/mocks/MetricsInterface.go

@@ -7,6 +7,8 @@ package mocks
 import (
 	logr "github.com/mattermost/logr"
 	mock "github.com/stretchr/testify/mock"
+
+	model "github.com/mattermost/mattermost-server/v5/model"
 )
 
 // MetricsInterface is an autogenerated mock type for the MetricsInterface type
@@ -61,7 +63,7 @@ func (_m *MetricsInterface) IncrementChannelIndexCounter() {
 }
 
 // IncrementClusterEventType provides a mock function with given fields: eventType
-func (_m *MetricsInterface) IncrementClusterEventType(eventType string) {
+func (_m *MetricsInterface) IncrementClusterEventType(eventType model.ClusterEvent) {
 	_m.Called(eventType)
 }