Provide onScaleStart and onScaleEnd callbacks

[ramtinq]

Thank you for this great and useful package.

I think some people may need a way to detect when zooming is being started, updated, and ended, so that they can update ui based on it; like changing the visibility of a bottom-navigation-bar.

So, these optional arguments could be added to the widget:

ZoomOverlay(
                onScaleStart: () {
                  ref.read(navigationBarHiddenProvider.notifier).state = true;
                },
                onScaleEnd: () {
                  ref.read(navigationBarHiddenProvider.notifier).state = false;
                },