Break interface definitions into their own files

[rocketstack-matt]

Feature Request

Description of Problem:

Interfaces enable us to describe how nodes can be interacted with. These could be well known interfaces that we will publish centrally (e.g. host/post) or could be bespoke to an organisation and will only be published within a firm.

At present, public interface definitions are in the same schema file as the meta schema for interfaces themselves which can be confusing and makes it less intuitive that it is ok to propose new interfaces for public use.

Potential Solutions:

Break the interface definitions into their own files published in a new interface folder below the meta folder.