Show user feedback dialog when a screenshot is detected

### Problem Statement

I'd like an option to automatically show the user feedback dialog whenever a user makes a screenshot of the app.

The use case is probably more for test builds than release builds, I imagine.

This enables a smoother experience when doing bug reports whenever there's no button for giving feedback on the current screen.

### Solution Brainstorm

1. Add screenshot detection to the underlying native SDKs
2. Pass a callback through to the Flutter SDK
3. Show user feedback dialog when the callback is executed
4. Profit 💰

I'd be willing to make the Dart/Flutter part happen

### Are you willing to submit a PR?

None