Add callbacks

[Swimburger]

Issue Summary

Twilio uses a lot of callbacks and webhooks.
It would be very helpful if the structure of these callbacks/webhooks was properly documented in the spec so we know what values to expect when generating code or writing code by hand.

https://swagger.io/docs/specification/v3_0/callbacks/

[tiwarishubham635]

Hi! I can communicate this to our teams for including it in the spec files (DII-1967). Thanks!

[Swimburger]

Even if you don't use the official syntax for callbacks and webhooks, just including the schemas would be massive.

[Akkuma]

Unfortunately, 6 months later this is still missing the webhooks as far as I can see.