RPC Processor

Thought this was missing. This is still a WIP though.

- [x] Add the RPC Processor
- [ ] Add the README
- [ ] Add some mofo tests
- [ ] Gather some feedback

I'm not really sure on some points though (just trying to adapt from [the official documentation](https://www.rabbitmq.com/tutorials/tutorial-six-php.html) and thought it could be useful here)