[Enhancement] Add an async initialization to Regions

[ramtechjoe]

Summary

The current API has an IInitializeAsync which provides a convenient way to run async code during page initialization. However, there does not exists a similar implementation pattern if async initialization is needed for a region ViewModel

API Changes

Could possibly add an IRegionAwareAsync interface which implements:

IRegionAwareAsync
{
    Task OnNavigatedToAsync(NavigationContext navigationContext)
}

This would need a supporting call from the MvvmHelpers class

Intended Use Case

Provide a clean approach to implementing async code during initialization of a region view model

[dansiegel]

@ramtechjoe there are 2 ways to implement this:

1. We wait for the task before loading the view.. this would lead to a really awful user experience
2. We invoke this after the view loads. I'm unclear how this would really be of any benefit to you besides hiding async void. Keep in mind that these methods are effectively method handlers and async void is actually ok here.