Restore MQTT functionality (if needed)

At present the CLI MQTT commands are missing. They were located in the tapir/cmd/mqtt.go file, but that was removed as the FetchMqttValidatorKey() function conflicted with the new, nodeman based, infrastructure for signing keys.

To restore, retrieve a version of tapir/cmd/mqtt.go from before it was removed (~March 2025) and update the code to work again.
