Provide a NavigatorObserver to track navigations

## Summary
In order to track navigations, a `NavigatorObserver` is used on Flutter but I've noticed that there isn't a `NavigatorObserver` provided by this package. Could we create a `NavigatorObserver` that observes navigations and tracks them using Amplitude? Check out [this](https://github.com/la-haus/flutter-segment/blob/master/lib/src/segment_observer.dart) code in `flutter-segment` repo for an example.

## Motivations
Providing a `NavigatorObserver` to track navigations using Amplitude.