Add async callback onStopAsync

[devsdocs]

Useful if the callback required an async operation, not perfect but just a simple idea.

[HeySreelal]

Hey!! Thanks for the contribution and the idea!

But, I think it's better to make the current _onStop method to accept something like FutureOr<void> Function(), so we don't have to add a separate method to do the same, right? Let me know what you think :)

[devsdocs]

Agree! Even better.

[HeySreelal]

Perfect, I'll wait for the updated PR - if you're on this :)