maui: Decide on the namespace of Authgear.Maui

[roxk]

Right now due to simple code sharing (and for back-compat), even in Authgear.Maui, the namespace for the classes is still Authgear.Xamarin.

• Decide what the new namespace should be for MAUI
• Decide a policy for future SDK versions (e.g. Uno)
• Implement different namespace guard as following

#if Xamarin
namespace Authgear.Xamarin {
#elif Maui
namespace Authgear.Maui {
#elif Uno
namespace Authgear.Uno {
else
// Assert error
#end