MQTT 5 Support

[gdsrmygdsrjr]

Some news about when will there be support for MQTT 5?

[bertmelis]

I once started working on MQTT5 but it is on the bottom of the to do list. So sorry, no ETA whatsoever.
I find it very complicated for a protocol that is aimed at constrained devices.

Progress can be tracked here: https://github.com/bertmelis/espMqttClient/tree/mqtt5
(or you're welcome to use the branch as a starting point for further development)