feat(openapi): export all oas types

[echozio]

I'm working on a project where I need to pass around parts of an OpenAPI spec a lot, and either need
to redefine these types or duplicate a lot of logic working from the top-level OpenApi object
when, for instance, I'm only interested in a single PathItem.

Exporting these would be useful for these types of situations, and since they adhere to and are
versioned according to the OAS, not to mention indirectly exposed through the OpenApi type, I
imagine these are fairly unlikely to see breaking changes.

[jdesrosiers]

Thanks!