Improve validation of generator input

[kierans]

The current POC of the generator combines validation with transformation which isn't the best idea.

Validation should be broken into a separate phase and not only checks that the input is valid against the various JSON schemas that define the Open API spec, but also that semantics of the spec makes sense, for example, do $refs resolve correctly.

In order to perform schema validation correctly, there needs to be a list of the various schemas used and how they're comprised as schemas extend/consume other schemas.

The first version of the validator should target Open API versions 3.0.x and 3.1.x while being extendable to add Swagger 2 support