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/channels/app/platform/websocket_router.go

@@ -73,6 +73,25 @@ func (wr *WebSocketRouter) ServeWebSocket(conn *WebConn, r *model.WebSocketReque
 		return
 	}
 
+	if r.Action == string(model.WebsocketPresenceIndicator) {
+		if chID, ok := r.Data["channel_id"].(string); ok {
+			// Set active channel
+			conn.SetActiveChannelID(chID)
+		}
+		if teamID, ok := r.Data["team_id"].(string); ok {
+			// Set active team
+			conn.SetActiveTeamID(teamID)
+		}
+
+		resp := model.NewWebSocketResponse(model.StatusOk, r.Seq, nil)
+		hub := conn.Platform.GetHubForUserId(conn.UserId)
+		if hub == nil {
+			return
+		}
+		hub.SendMessage(conn, resp)
+		return
+	}
+
 	if !conn.IsAuthenticated() {
 		err := model.NewAppError("ServeWebSocket", "api.web_socket_router.not_authenticated.app_error", nil, "", http.StatusUnauthorized)
 		returnWebSocketError(conn.Platform, conn, r, err)