OpenAPI spec callbacks

[jayvdb]

OpenAPI callbacks define outgoing async requests, typically back to the sender.
docs: https://swagger.io/docs/specification/callbacks/
spec: https://spec.openapis.org/oas/v3.0.2#callbackObject

https://blog.stoplight.io/mocking-callbacks-openapi-prism shows how these callbacks are utilised by https://github.com/stoplightio/prism

It would be great if the OpenAPI support included support for these in the same manner.