OpenAPI spec callbacks

[jayvdb]

The OpenAPI support is experimental, so it isnt surprising that OpenAPI spec callbacks are not supported yet.

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.

It looks like the mockintosh native config yaml also does not support this type of async HTTP callback, but I would love to be proven wrong about that.