Add support for OAS 3 callback documentation

[richiksc]

Is your feature request related to a problem? Please describe.
Add support for documenting callbacks, which are in OpenAPI specification v3 - https://swagger.io/docs/specification/callbacks/. Some services I'm documenting rely heavily on callbacks and are a common pattern in webhooks and publish/subscribe pattern.

Describe the solution you'd like
Add support for documenting callbacks through, for example, a @callback annotation on an endpoint.

/*
 * @callback {object} eventName - description - application/json
 * Response body (JSON)
 */