Support for openapi Schema type definitions

[fprochazka]

We will probably need to support two strategies for the OpenApi schema definitions:

• raw types - String with regexp for UUID, and Number for the bigint, which is actually how the values are serialized and transferred over HTTP
• domain types - specific shapes for the values - this will be harder to do, as this has no equivalent on the HTTP JSON layer, and a discovery of how this would work in the generated clients needs to be concluded first

[fprochazka]

I've learned, after working on #17, that OpenAPI Generator ignores named types if they have a primitive type - so the ID types are preserved in the OpenAPI spec, but not in the generated client. This is not ideal, but understandable, and should be fixable with a custom generator.