MM-56060: Create base scaffolding for websocket pub-sub (#25654)

We create a new websocket action called "presence" which
can contain the active_channel and the active_team for a given
client connection.

On the client side, for every channel or team switch, we send
out this message.

https://mattermost.atlassian.net/browse/MM-56060

```release-note
NONE
```

server/public/model/websocket_message.go

@@ -86,6 +86,7 @@ const (
 	WebsocketEventAcknowledgementRemoved              WebsocketEventType = "post_acknowledgement_removed"
 	WebsocketEventPersistentNotificationTriggered     WebsocketEventType = "persistent_notification_triggered"
 	WebsocketEventHostedCustomerSignupProgressUpdated WebsocketEventType = "hosted_customer_signup_progress_updated"
+	WebsocketPresenceIndicator                        WebsocketEventType = "presence"
 )
 
 type WebSocketMessage interface {