fix: makes schema subtypes public, adds oneOf field support

[josh-burton]

Thanks for this great package. It seems one of the most complete in terms of parsing and working with OpenApi specs.

I'm developing my own generators and using this package primarily for parsing. I hope you're open to merging these small changes.

• adds oneOf field support
• makes some of the freezed union subtypes public. This allows me to differentiate on the type of schema or parameter by type.

[walsha2]

Merged in #78 - Thanks for the PR!

This repo (as soon as I get time) will get a major overhaul (e.g. #73) and other breaking changes to make it easier to handle all the edge cases and nuances of the OpenAPI spec. Just a heads up! If you have any input on generation, feel free to provide it.