Add option to pass a custom `screenshotController`

[TarekMedhat]

:scroll: Description

Add the option to pass a custom ScreenshotController

:bulb: Motivation and Context

I had a use case that required custom capture logic, which let me add native screenshot support for platform views by capturing the view natively.

:green_heart: How did you test it?

Implemented a custom ScreenshotController and passed it to BetterFeedback widget.

:pencil: Checklist

• [x] I reviewed submitted code
• [x] I added tests to verify changes
• [x] I updated the docs if needed
• [x] All tests passing
• [x] No breaking changes

:crystal_ball: Next steps

🔁 : Alternative Solution:

Another possible solution: is to only make the ScreenshotController key public and static without any further changes. static final GlobalKey containerKey = GlobalKey(); This works since the feedback widget wraps the app, so there will always be one scrollController all the time. Then feedbackBuilder can be used to override the onSubmit logic.