Expose the platform window creation on WinUI and Catalyst so users can subclass

[PureWeen]

Description

Currently it's not possible to subclass UIWindow or UI.Xaml.Window

Can we expose the creators for these as PlatformViewFactories like we do on all the other handlers?

https://github.com/dotnet/maui/blob/main/src/Core/src/Platform/iOS/ApplicationExtensions.cs#L93-L97

maui/src/Core/src/Platform/Windows/ApplicationExtensions.cs

Line 16 in e4124ab

var winuiWindow = new MauiWinUIWindow();

iOS Notes

Right now on iOS we just create a UIWindow we should probably return a MauiUIWindow if we make it so users can set their own UIWindow.

[bdovaz]

Any updates?