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_client.go

@@ -326,6 +326,22 @@ func (wsc *WebSocketClient) GetStatusesByIds(userIds []string) {
 	wsc.SendMessage("get_statuses_by_ids", data)
 }
 
+// UpdateActiveChannel sets the current channel that the user is viewing.
+func (wsc *WebSocketClient) UpdateActiveChannel(channelID string) {
+	data := map[string]any{
+		"channel_id": channelID,
+	}
+	wsc.SendMessage(string(WebsocketPresenceIndicator), data)
+}
+
+// UpdateActiveTeam sets the current team that the user is in.
+func (wsc *WebSocketClient) UpdateActiveTeam(teamID string) {
+	data := map[string]any{
+		"team_id": teamID,
+	}
+	wsc.SendMessage(string(WebsocketPresenceIndicator), data)
+}
+
 func (wsc *WebSocketClient) configurePingHandling() {
 	wsc.Conn.SetPingHandler(wsc.pingHandler)
 	wsc.pingTimeoutTimer = time.NewTimer(time.Second * (60 + PingTimeoutBufferSeconds))

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 {