[Android] [iOS] In-app screen sharing (only a part of own app)

[WillowWisp]

Is your feature request related to a problem? Please describe.
Sometimes, during presentations, hosts might want to only share a part of the screen to avoid their personal data being revealed (eg. push notifications).

Describe the solution you'd like
I think it's better to allow sharing only the content of the app (and being able to choose which content to share (either by widget or by screen region) would be amazing).

Describe alternatives you've considered
I'm not familiar with native side so I don't know if this is technically possible on Android & iOS, but it seems like flutter_webrtc can do something similar so I think it's worth taking this into consideration.