Support for user properties

[cozydvlpr]

FEATURE REQUEST

1. No, there's no other issue or PR on this

2. Allow to inspect user properties in the MQTT PUBLISH/SUBSCRIBE message and eventually change them.
   (Moreover, currently mproxy sends an incorrect packet to the upstream MQTT proxy. Try for example the following: mosquitto_pub -m "hello1" -t "this_is_hello" -h localhost -p 1883 --property PUBLISH user-property shop 'storeName:mall'. )

3. It's important for my project to intercept user properties to remove or add them depending on the environment where the proxy runs (production, test, etc).

[drasko]

User properties are part of MQTT 5 standard (https://www.hivemq.com/blog/mqtt5-essentials-part6-user-properties/, http://www.steves-internet-guide.com/examining-mqttv5-user-properties/).

mProxy supports only MQTT 3.1, and we currently do not plan to add support for MQTT 5 in a very short term (it is on the roadmap, but will not happen soon).

The only way to proceed quickly at this point would be either professional support (a feature request through Abstract Machines company that does professional support for the Mainflux project) or you will have to do your changes on your own (probably it will not be that complicated, you can take an example of the existing handlers).