Add support for custom barriers on ModalSheet

[bqubique]

Add support to add custom barrier widget instead of the standard one.

[fujidaiti]

Could you please explain what this feature is for?

[bqubique]

Hey, I will have a look at it during the weekend @fujidaiti. Basically it adds support to have any kind of widget instead of AnimatedBarrier only. For example I added an ImageFilter which blurred the background of the modals.
I will need to handle other modals as well, as this only works for the GoRoute Shells. Will continue working on it hopefully soon!

[fujidaiti]

I think this should be a builder rather than a widget because the barrier is lazily built in buildModalBarrier. The signature could be something like ModalSheetBarrierBuilder? get barrierBuilder where ModalSheetBarrierBuilder is:

typedef ModalSheetBarrierBuilder = Widget Function(PageRoute route, VoidCallback onDismissCallback);

The route parameter is necessary to get barrierColor, barrierDismissible, etc...