add support for MQTT 3.1

[Freax13]

This pr adds support for MQTT 3.1 to rumqttc.

This pr is based on https://www.oasis-open.org/committees/download.php/55095/mqtt-diffs-v1.0-wd01.doc.

This pr introduces a breaking change:

rumqtt/rumqttc/src/mqttbytes/v4/connect.rs

Line 22 in 23b23d6

pub fn new<S: Into<String>>(protocol: Protocol, id: S) -> Connect {

[Freax13]

The failed CI job seems unrelated to this pr.

[de-sh]

Hey @Freax13, had a discussion about this PR with @tekjar. We are currently blocked on adding MQTT 5 support, so we are not very sure about how we could maintain support for 3.1 going ahead. We are curious about where and why you continue using it, instead of 3.1.1 or 5.

[Freax13]

We are curious about where and why you continue using it, instead of 3.1.1 or 5.

We don't have a choice, we don't control the broker/server we're connecting to.

[de-sh]

@Freax13 we are sorry, but the added cost of maintaining this is not something we are ready to take up atm.