flash method
Flash data to session
Implementation
@override
Future<void> flash(String sessionId, String key, dynamic value) async {
final data = await getSession(sessionId);
if (data == null) return;
data['_flash'] ??= <String, dynamic>{};
(data['_flash'] as Map<String, dynamic>)[key] = value;
await _currentDriver.write(sessionId, data);
}