Motivation

It's helpful for a client to know what was the exact reason for being disconnected by a server. MQTT has an extensive reason code list for this. Currently there does not seem to be a way for MQTTX to show this reason code.

Detailed design

When a connection is terminated provide a way to see the reason.

Alternatives

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More detail (optional)

The reason code list from the MQTT 5 specification can be found here