getDehydratedDevice method
fetch a dehydrated device. https://github.com/matrix-org/matrix-spec-proposals/pull/3814
Implementation
Future<DehydratedDevice> getDehydratedDevice() async {
  final response = await request(
    RequestType.GET,
    '/client/unstable/org.matrix.msc3814.v1/dehydrated_device',
  );
  return DehydratedDevice.fromJson(response);
}