about NavigationAppBar's default leading

[frg2089]

Is your feature request related to a problem? Please describe.
I want to override the onPressed behavior of NavigationAppBar's default leading, but NavigationAppBar does not provide the corresponding interface implementation.

Describe the solution you'd like
Provides an interface to override the onPressed behavior of default leading.
Or remove the Padding from _buildLeading .
Or allow me to pass in a GlobalKey<NavigatorState> to be used to modify the default behavior.

Describe alternatives you've considered
I tried to copy the default leading into my project, but _buildLeading adds a Padding to it causing the button to look weird.

Additional context
https://github.com/bdlukaa/fluent_ui/blob/master/lib/src/controls/navigation/navigation_view/view.dart#L702-L706