[Enhancement] Add an async initialization to Regions

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