[Spec] add petstore-3.0-native.yaml spec with oneOf request schema

[grokify]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

As more specs start to use OpenAPI 3.0 native features, it is becoming more important to test them.

This PR adds a petstore-3.0-native.yaml spec to contain native OpenAPI 3.0 spec artifacts, like oneOf, as opposed to be converted from a 2.0 spec. This is based on resources/3_0/petstore.yaml and currently contains the additional PATCH /pets endpoint as seen below. More will be added to this spec so we can have a test spec as we build out OAS3 features.

https://swagger.io/docs/specification/data-models/oneof-anyof-allof-not/

This has been validated here:

https://mermade.org.uk/openapi-converter

paths:
  /pets:
    patch:
      requestBody:
        content:
          application/json:
            schema:
              oneOf:
                - $ref: '#/components/schemas/Cat'
                - $ref: '#/components/schemas/Dog'
      responses:
        '200':
          description: Updated
components:
  schemas:
    Dog:
      type: object
      properties:
        bark:
          type: boolean
        breed:
          type: string
          enum: [Dingo, Husky, Retriever, Shepherd]
    Cat:
      type: object
      properties:
        hunts:
          type: boolean
        age:
          type: integer

[wing328]

@grokify thanks for the PR. Let me check the license of the spec first before merging it into our repo.

[wing328]

More tests added to the folder: https://github.com/OpenAPITools/openapi-generator/tree/master/modules/openapi-generator/src/test/resources/3_0

Closing this one.