Allow or expose scroll controller to be used outside.

[CodeWizard0367]

I have complex widget where I need to use scroll controller's position at the same time I also need to use the scroll to index functionality with current implementation either I can use Listview with scroll controller to use scroll controller or I can use scrollable positioned list but in that case I am not able to use scroll controller.

It would be very helpful if we can pass the scroll controller from outside to utilise it in other places as well.