Add the capability to passe a NavigatorKey to Flyout

[WinXaito]

In my app, I want to open a flyout with a context that doesn't have a Navigator (in the appbar, and the appbar is not in the navigator context). I use stacked and work with the service NavigatorService (stacked/stacked_services).

Example:

As I shown in the discord:

And the navigator is inside the blankpage.