Actor to give to Actix to do the background processing of NATS messages/requests
PublishMessage is a message that publishes a buffer to the NATS queue but doesn't wait for a reply afterwards. Think of it as the Pub part in PubSub.
Subscribe is a message that subscribes to a topic in the NATS queue Think of it as the Sub part in PubSub. RequestWithReply creates a reply inbox in NATS, publishes a message (Request in terms of NATS grammar) and waits for the reply to come back.