[Featurerequest] Implementing MQTT 5.0

[godofglow]

With the release of the MQTT 5.0 specification Link there is a upcoming product environment and some useful features.
Should be possible to select between 3.1.1 and 5.0.

[mwallnoefer]

Looking at the commit log https://github.com/eclipse/paho.mqtt.m2mqtt/commits/master/ it seems that the library is basically dead. But it would be important to mention that in an official way.