MM-57152: Get webconn count from the whole cluster (#26813)

We were setting the user status to offline without
checking for connections on other nodes in a cluster.

Now we implement a request-response mechanism for the whole
cluster and we check that before setting a user to offline.

https://mattermost.atlassian.net/browse/MM-57153

```release-note
Fix a bug where the user status would incorrectly be set to offline
without checking for connections in other nodes in an HA cluster.
```

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>
Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/public/model/cluster_message.go

@@ -51,6 +51,8 @@ const (
 	ClusterGossipEventResponseGetPluginStatuses = "gossip_response_plugin_statuses"
 	ClusterGossipEventRequestSaveConfig         = "gossip_request_save_config"
 	ClusterGossipEventResponseSaveConfig        = "gossip_response_save_config"
+	ClusterGossipEventRequestWebConnCount       = "gossip_request_webconn_count"
+	ClusterGossipEventResponseWebConnCount      = "gossip_response_webconn_count"
 
 	// SendTypes for ClusterMessage.
 	ClusterSendBestEffort = "best_effort"