Modules
An embeddable MQTT 3.1.1 / 5.0 broker: for tests, edge devices, or an app that is its own broker.
An MQTT 3.1.1 / 5.0 client.
A PUBLISH delivered to a subscription, sent to the client's owner as
{:stem_mqtt, client.id, {:message, %StemMqtt.Message{}}}.