Implement missing MQTTv3 protocol support

[canni]

According to docs MQTTv3 should be supported, but the Go SDK lacks v3 support.

Probably thats's because paho has 2 confusingly named libraries:

• github.com/eclipse/paho.golang - used currently by the SDK, it only supports MQTT v5
• github.com/eclipse/paho.mqtt.golang - older client that supports MQTT v3

This PR implements proto that would support v3 servers, tested with mochi broker.

Any feedback welccome!