sendInvitation method
Implementation
@override
void sendInvitation(String username, String? invitationId) {
InvitationMessage message = InvitationMessage(
id: invitationId ?? const Uuid().v4(),
type: MessageType.invitationRequest,
sendTime: DateTime.now().millisecondsSinceEpoch);
String topic =
username.toInvitationEventTopic; //TODO: decide the right topic
clientHandler.sendPayload(message.toJson().toString(), topic);
}