Callbacks/webhooks support

Hi, and thanks for all the great work on this package.

[OpenAPI 3.x](https://swagger.io/docs/specification/callbacks/) / [Swagger](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#callbackObject) supports callbacks, but I can't seem to find any docs or code related to that in Swashbuckle. I have an ASP.NET Core API with webhooks that I'd like to use with Swashbuckle and display in the Swagger UI page.

- Does Swashbuckle.AspNetCore support callbacks?
- If so, how can I set that up? Using attributes, XML documentation, etc.?
- If not, I'd like to make a feature request for it.

PS: Let me know if this is belongs on StackOverflow instead of here as per the contributing guidelines, and I'll move it.

Review the linked OpenAPI 3.x and Swagger callback specifications first, then inspect Swashbuckle.AspNetCore's existing API-documentation generation and Swagger UI integration. Done would mean callbacks are supported for ASP.NET Core APIs and the supported setup method is documented.

Written by the indexing model from the issue text.