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/einterfaces/cluster.go

@@ -29,4 +29,7 @@ type ClusterInterface interface {
 	QueryLogs(page, perPage int) (map[string][]string, *model.AppError)
 	GetPluginStatuses() (model.PluginStatuses, *model.AppError)
 	ConfigChanged(previousConfig *model.Config, newConfig *model.Config, sendToOtherServer bool) *model.AppError
+	// WebConnCountForUser returns the number of active webconn connections
+	// for a given userID.
+	WebConnCountForUser(userID string) (int, *model.AppError)
 }