Make it possible to use a background `URLSessionConfiguration`

[fousa]

This is useful when you want to send analytics to your own server, and you want this to happen in the most performant way.

Make sure that the completion handler can be triggered from the UIApplicationDelegate.

func application(_ application: UIApplication,
                 handleEventsForBackgroundURLSession identifier: String,
                 completionHandler: @escaping () -> Void)