[Core] oneOf schema properties

Created by: lwlee2608

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

vars missing from model if they are given oneOf rule

openapi-generator version

4.2.2-SNAPSHOT

OpenAPI declaration file content or url

    Area:
      type: object
      oneOf:
        - required:
          - tacs
        - required:
          - areaCode
      properties:
        tacs:
          type: array
          items:
            $ref: '#/components/schemas/Tac'
          minItems: 1
        areaCode:
            $ref: '#/components/schemas/AreaCode'

Steps to reproduce

checkout from here ./run.sh

The generated code won't compile because Area is missing properties tacs and areaCode

Suggest a fix

call addVars from schema.properties here https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java#L1939