Support OpenAPI 3.1.0

[simplesmiler]

Motivation

• Upcoming release of Swagger v4, with both UI and Editor supporting OpenAPI 3.1.0
• OpenAPI 3.1.0 introduces a proper null type, which nicely maps to TypeScript null type. Prior to 3.1.0 there was nullable field, but it is very inconvenient at times, e.g. Reference objects don't combine well with “nullable” OAI/OpenAPI-Specification#1368

Spec: https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.0.md and https://spec.openapis.org/oas/v3.1.0
3.0 -> 3.1: https://www.openapis.org/blog/2021/02/16/migrating-from-openapi-3-0-to-3-1-0

Blockers:

• Add support to openAPI 3.1.* APIDevTools/swagger-parser#169 and zSchema doesn't support json-schema post 2020-12 zaggino/z-schema#268, we use that to fetch, validate and parse
  • Fixed upstream, z-schema is not relevant anymore.
• swagger2openapi, we use that to convert to the latest version (currently 2.x -> 3.0) to simplify further processing.
  • Not really a blocker for 3.1.

[crutch12]

Waiting for Mermade/oas-kit#529

[simplesmiler]

APIDevTools/swagger-parser#187 landed in 10.1.0, so parser stopped complaining.

[simplesmiler]

In that PR z-schema was replaced with ajv, so it's not a blocker anymore.

[simplesmiler]

swagger2openapi and oas-kit issues seem to be related to up-conversion to 3.1 (e.g. #25) rather than being blockers to 3.1 support.

[simplesmiler]

Committed @apidevtools/swagger-parser@^10.1.0 to the 0.2.x and master branches.
With this, OpenAPI 3.1 is supported in principle, and this issue can be closed.

If there are specific problems with specific examples, they can be tackled in specific issues.