A PUBLISH delivered to a subscription, sent to the client's owner as {:stem_mqtt, client.id, {:message, %StemMqtt.Message{}}}.
{:stem_mqtt, client.id, {:message, %StemMqtt.Message{}}}
@type t() :: %StemMqtt.Message{ payload: binary(), qos: 0..2, retain: boolean(), topic: String.t() }