Distil Open API spec 3 & 3.1 into common set of abstractions

This is currently a very ambiguous issue. However the goal is to know that the generator can process any OAS 3 or 3.1 spec. To do that, a set of abstractions around the AST the generator uses internally will need to be produced, along with tests to ensure that an API spec can be parsed (with all different variations the spec allows) into an AST that can then be used to generate code.