Add OpenAPI v3.1 importer

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

Add support for OpenAPI 3.1 collections, which are not compatible with OpenAPI 3.0. This pulls in a new library oas3 because openapiv3 only supports 3.0. The openapi format for slumber import will check the version of the given OpenAPI file so we didn't have to add a new target for this.

Closes #513

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

• This certainly has bugs. There's some edge cases where I cut corners, ambiguity in the OpenAPI library's types, and inherent incompatibilities between OpenAPI and Slumber. I did my best and we'll have to patch bugs as they're reported
• This can't be merged until this oas3 PR is merged

QA

How did you test this?

New integration test

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked