Callback invoked when pulled by refreshTriggerPullDistance.
If provided, must return a Future which will keep the indicator in the
RefreshIndicatorMode.refresh state until the Future completes.
Can be null, in which case a single frame of RefreshIndicatorMode.armed
state will be drawn before going immediately to the RefreshIndicatorMode.done
where the sliver will start retracting.
final RefreshCallback onRefresh