Add Message Receiver contract oracle bridge

## 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](https://app.mural.co/t/consensys3989/m/consensys3989/1557345438344/718181f0ee878f1cfdc2756c9dc6aa363680b634)
- [Previous Issue](https://github.com/ConsenSys/web3studio-sidejam/issues/5)

### 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