MM-56876: Redis: first introduction (#27752)

```release-note
NONE
```

---------

Co-authored-by: Jesús Espino <jespinog@gmail.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/cluster_message.go

@@ -6,6 +6,7 @@ package model
 type ClusterEvent string
 
 const (
+	ClusterEventNone                                        ClusterEvent = "none"
 	ClusterEventPublish                                     ClusterEvent = "publish"
 	ClusterEventUpdateStatus                                ClusterEvent = "update_status"
 	ClusterEventInvalidateAllCaches                         ClusterEvent = "inv_all_caches"