[Flutter] We should be able to customize the animations for the Navigators used by Beagle

[Tiagoperes]

Use case

I'd like to setup custom animations for pushViews, pushStacks, popViews, etc. Currently, we can't customize any animation for navigations.

Proposal

The RootNavigator should receive an object that implements the AnimatableNavigator interface. This interface should have methods like animatePushView, animatePushStack, etc. In truth, this interface should be based on how navigator animations work on Flutter.