Feature Request: Support Open API callbacks

[twastvedt]

We'd like to be able to describe callbacks/webhooks in our API. For example,

paths:
  /subscribe:
    post:
      summary: Subscribe to a webhook
      requestBody:
        …
      callbacks:   # Callback definition
        myEvent:   # Event name
          '{$request.body#/callbackUrl}':   # The callback URL,
                                            # Refers to the passed URL
            post:
              requestBody:   # Contents of the callback message
                required: true
                content:
                  application/json:
                    schema:
                      type: object
                      properties:
                        message:
                          type: string
                          example: Some event happened
                      required:
                        - message
              responses:   # Expected responses to the callback message
                '200':
                  description: Your server returns this code if it accepts the callback

(https://swagger.io/docs/specification/callbacks)

For the above example, Oazapfts generates the /subscribe endpoint, but nothing is generated for the callback part of the spec. Ideally we would get a type for the body of the callback request, something like:

type myEventBody = {
  message: string
}

I'm not sure if there's anything beyond that which would be useful to generate?