The onTap function is called when a user taps on CupertinoListTile. If
null, the CupertinoListTile will not react on taps. If this is a
Future<void> Function(), then the CupertinoListTile remains activated
until the returned future is awaited. This is according to iOS behavior.
However, if this function is a
void Function(), then the tile is active
only for the duration of invocation.
final FutureOr<void> Function()? onTap;