Create an Unresolved* typealias for all supported OpenAPI types

[czechboy0]

For easier handling of potentially referenceable OpenAPI types in the generator, e.g. Either<JSONReference<JSONSchema>, JSONSchema>?, we should create a typealias called Unresolved* (e.g. UnresolvedSchema). This is already present for some of the types, but not all.

The full list right now is:

• schemas
• parameters
• (response) headers
• request bodies
• responses