Native Geofencing Capabilities

Hello,

I just discovered this project and am amazed! It is amazing.
Looking at the implementation of the geofencing, I can see that the `carp_context_package` relies on the `location` package and its `onLocationChanged` method. This means geofencing will drain much more battery than necessary as it could utilize the native capabilities of Android and iOS.
I was looking into other packages that allow my app to react to geofencing events if when the app is fully terminated (killed in task switcher) and this would only be possible if the native geofencing is used. From a study and research perspective this could allow a more granular start and stop behavior of a study to avoid draining the battery throughout the day.

Here is the android documentation: https://github.com/cph-cachet/carp.sensing-flutter/tree/version-0.40.x/apps/carp_mobile_sensing_app
And here is iOS: https://developer.apple.com/documentation/corelocation/monitoring_the_user_s_proximity_to_geographic_regions