Version Switch Observer
An interface which enables listening to navigation tiles version switch. Navigator might be switched to a fallback tiles version when there are no enough tiles of the current version to navigate on. It might happen when network is not available and tiles can't be loaded. When connection is restored, navigator will switch back to the target version specified in RoutingTilesOptions (the latest available version if no version is specified). To create additional fallback tiles versions, use the TileStore to create and download offline regions. Otherwise, fallback candidates will only be the tiles versions from the ambient cache.