Allow Bridging from existing MQTT Brokers to internal service

**Is your feature request related to a problem? Please describe.**
I like the idea of the integrated mqtt service. But I already have configured all my devices to a dedicated broker (mosquitto). I am now looking for an easy way to bridge my existing broker to the integrated mqtt service.

**Describe the solution you'd like**
Allow bridging from other brokers like e.g. Mosquitto

**Describe alternatives you've considered**
Create a dedicated client that subscribes to # and forwards all messages to IOTDB and vice versa.

**Additional context**
Mosquitto.conf:
persistence true
persistence_location /mosquitto/data/
log_dest file /mosquitto/log/mosquitto.log

connection iotdbltbridge
address 192.168.1.13
topic #
remote_username root
remote_password ****

When trying to setup a bridge on mosquitto side I get an exception on IOTDB side:
```
021-05-27 09:40:58,357 [nioEventLoopGroup-3-5] ERROR i.m.b.NewNettyMQTTHandler:87 - Unexpected exception while processing MQTT message. Closing Netty channel. CId=null
java.io.IOException: invalid massage
at io.moquette.broker.NettyUtils.validateMessage(NettyUtils.java:86)
at io.moquette.broker.metrics.MQTTMessageLogger.logMQTTMessage(MQTTMessageLogger.java:61)
at io.moquette.broker.metrics.MQTTMessageLogger.logMQTTMessageRead(MQTTMessageLogger.java:50)
at io.moquette.broker.metrics.MQTTMessageLogger.channelRead(MQTTMessageLogger.java:45)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.moquette.broker.metrics.MessageMetricsHandler.channelRead(MessageMetricsHandler.java:50)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:310)
at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:297)
at io.netty.handler.codec.ReplayingDecoder.callDecode(ReplayingDecoder.java:350)
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:265)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.channel.ChannelInboundHandlerAdapter.channelRead(ChannelInboundHandlerAdapter.java:86)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.channel.ChannelInboundHandlerAdapter.channelRead(ChannelInboundHandlerAdapter.java:86)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:286)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.moquette.broker.metrics.BytesMetricsHandler.channelRead(BytesMetricsHandler.java:51)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:340)
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1414)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:362)
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:348)
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:945)
at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:146)
at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:645)
at io.netty.channel.nio.NioEventLoop.processSelectedKeysPlain(NioEventLoop.java:545)
at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:499)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:459)
at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:884)
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
at java.base/java.lang.Thread.run(Unknown Source)
Caused by: io.netty.handler.codec.mqtt.MqttUnacceptableProtocolVersionException: MQIsdp and -125 are not match
at io.netty.handler.codec.mqtt.MqttVersion.fromProtocolNameAndLevel(MqttVersion.java:55)
at io.netty.handler.codec.mqtt.MqttDecoder.decodeConnectionVariableHeader(MqttDecoder.java:216)
at io.netty.handler.codec.mqtt.MqttDecoder.decodeVariableHeader(MqttDecoder.java:182)
at io.netty.handler.codec.mqtt.MqttDecoder.decode(MqttDecoder.java:88)
at io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:489)
at io.netty.handler.codec.ReplayingDecoder.callDecode(ReplayingDecoder.java:367)
... 32 common frames omitted
2021-05-27 09:41:10,498 [nioEventLoopGroup-3-4] INFO i.m.broker.MQTTConnection:253 - Notifying connection lost event. CId: mqtt-explorer-57cdbe16, channel: [id: 0x7a76b80d, L:0.0.0.0/0.0.0.0:1883]
```

MQTTエンドポイントに対するMosquitto bridge接続を再現し、その後、NettyUtils.java、MQTTMessageLogger.java、MqttDecoderで示されている例外パスから着手します。既存のbrokerが正常に接続でき、外部brokerと統合サービスの間で両方向にメッセージが流れれば完了です。

索引モデルが issue の本文から書いたものです。