readFrom method
Creates a payload from the specified header stream.
Implementation
@override
void readFrom(MqttByteBuffer payloadStream) {
// The payload of the publish message is not a string, just
// a binary chunk of bytes.
// The length of the bytes is the length specified in the header,
// minus any bytes spent in the variable header.
final messageBytes = header!.messageSize - variableHeader!.length;
message = payloadStream.read(messageBytes);
}