[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

Start by locating the existing IInitializeAsync pattern, the region navigation lifecycle, and the MvvmHelpers class mentioned in the issue. Trace how synchronous region awareness is invoked and review related tests if present. Done means an agreed async region-initialization API is integrated into that lifecycle with coverage for the intended ViewModel use case.