Custom navigation controller

[joemasilotti]

This PR provides support to customize the UINavigationController used in Turbo Navigator.

This functionality was originally removed when we introduced TurboNavigationHierarchyController as it is internal to the library.

For context, I need a way to hide the status bar on certain screens. Normally, this would work:

class TurboWebViewController: VisitableViewController {
    override var prefersStatusBarHidden: Bool { true }

    // ...
}

But when presented in a UINavigationController this is ignored. The following change needs to be made to the parent navigation controller to defer this setting to the top view controller:

class NavigationController: UINavigationController {
    override var childForStatusBarHidden: UIViewController? { topViewController }
}

This PR allows me to configure the default navigation controllers, like so:

Turbo.config.defaultNavigationController = { NavigationController() }

[zoejessica]

Very nice, it's useful to have this customization point 👍