Add Message Receiver contract oracle bridge

[brian-lc]

Overview

As an oracle-ish system interacting with the messages bus, I would like a way to receive messages directly from the bus.

Reference

• Mural
• Previous Issue

Questions

• How will external services be registered?
• What additional logic is needed to process outbound messages?
• What happens when calls fail? Is there a retry?

Assumptions

• There will be one contract to handle all external systems

Acceptance

• it should make calls to external systems with the messages provided by the bus
• it can accept more than one message
• messages do not expire
• message topics can be re-used