Add option to pass a custom screenshotController

[TarekMedhat]

📜 Description

Add the option to pass a custom ScreenshotController

💡 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.

💚 How did you test it?

Implemented a custom ScreenshotController and passed it to BetterFeedback widget.

📝 Checklist

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

🔮 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.