Support external $refs in openapi schema

[markfejes]

In larger projects the openapi schema file is usually divided to more parts, but currently it's not handled in this package, which casues an ajv compilation error if there is any external $ref.

main.yml

components:
  schemas:
    UserList:
      type: array
      items:
        $ref: './user.yaml#/definitions/User'

user.yml

definitions:
  User:
    type: object
    properties:
      id:
        type: string
      name:
        type: string
    required: ['id']

Currently the external refs are not handled in @openapi-contrib/openapi-schema-to-json-schema https://github.com/openapi-contrib/openapi-schema-to-json-schema#features

NOTE: $refs are not handled in any way, so please use a resolver such as json-ref-resolver prior to using this package.

I've created a pull request which uses @apidevtools/json-schema-ref-parser to process external $refs.