[Feature Request] [Web Apps] Display loader while Web App is loading

[heyqbnk]

Hey! Here is a feature request connected with displaying placeholder during Web App loading. Not to duplicate the issue content across all Telegram applications, I provide link to the same issue in Telegram Desktop:

https://github.com/telegramdesktop/tdesktop/issues/25792

Please, let me know, if you want me to copy the issue content and place it here.