RPC Processors

Thought this was missing. This is still a WIP though, as I just want to gather some feedbacks first.

- [x] Add the RPC Server Processor
- [x] Add the RPC Client Processor
- [x] Add the README
- [x] Add some mofo tests

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)