readFrom method
Creates a variable header from the specified header stream.
Implementation
@override
void readFrom(MqttByteBuffer variableHeaderStream) {
  readTopicName(variableHeaderStream);
  if (header!.qos == MqttQos.atLeastOnce ||
      header!.qos == MqttQos.exactlyOnce) {
    readMessageIdentifier(variableHeaderStream);
  }
}