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]
```

Stelle die Mosquitto-Bridge-Verbindung zum MQTT-Endpunkt nach und beginne dann mit dem in NettyUtils.java, MQTTMessageLogger.java und MqttDecoder genannten Ausnahme-Pfad. Erledigt ist die Aufgabe, wenn sich ein vorhandener Broker erfolgreich verbinden kann und Nachrichten in beide Richtungen zwischen dem externen Broker und dem integrierten Dienst fließen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.