Use NATS for CoreWorkflow transmit step

[ccorley]

Investigate moving the transmit implementation out of CoreWorkflow (the transmit step would still remain in CoreWorkflow). Create a NATS subscriber that handles transmission of messages to defined transmit servers (right now FHIR servers) and returns the response via a) another NATS message or b) a callback.

Consider moving transmission settings to a .yaml file and make the settings more general.