private void sendSubMessage(final String topic, final String message, final int packetId, final Channel channel) {
        MqttFixedHeader fixedHeader = new MqttFixedHeader(MqttMessageType.PUBLISH, false, AT_MOST_ONCE, true, 0);
        MqttPublishVariableHeader varHeader = new MqttPublishVariableHeader(topic, packetId);
        MqttPublishMessage mqttPublishMessage = new MqttPublishMessage(fixedHeader, varHeader, Unpooled.copiedBuffer(message, CharsetUtil.UTF_8));
        channel.writeAndFlush(mqttPublishMessage);
    }
send subscribe message.
@param topic topic
@param message message
@param packetId packetId
@param channel channel
shenyu-protocol/shenyu-protocol-mqtt/src/main/java/org/apache/shenyu/protocol/mqtt/Subscribe.java