feat: Reference schema support (#800)

[goodsonjr]

This PR adds support for reference schemas (#800).

The idea with this change is that during parsing of the schemas, when the parser encounters a Reference schema, rather than aborting it will attempt to fully resolve that reference by following the chain of references until it encounters a Schema. When it does, it will attempt to fetch that parsed Schema from the dictionary of available components, which should include all possible referent schemas. If it exists, it will parse the current/original reference Schema as if it was its ultimate referent. If it reaches a reference that does not exist, it logs an error and moves on: it won't be able to parse this Reference.

The result of this is that every reference schema in the API doc will result in a separate-but-identical class, aside from the class name. This feels like a safe, basic way to handle the situation, since collapsing identical classes might break compatibility between the API and the codegen at some point. I suppose we could instead have references be subclasses or some trick like that to cut down on code duplication and let identical API objects be equivalent? If that would be preferable it is probably doable downstream of this change, since with this parsing the Schema objects added as references in the Schemas class should be identical objects which could be identified and collapsed in some fashion during code templating. Perhaps it would be useful to record the direction of references during parsing to better choose what a base class would be... That would require some more thought.

In any case, this works both for single references as well as arbitrarily deep nested references, something useful for the API I'm developing for.

Also includes some functional tests which include both a valid and invalid (circular reference) case for reference schemas and adjusts an existing unit test which assumed that the reference schema would be skipped to one that tests whether it has been parsed.

[dbanty]

Looks great to me, thanks!