Built-in Swagger API descriptor support on /swagger.yaml for Custom HST Plain REST APIs

As a developer, if an HST Plain REST API mount is configured at /restapi for instance, then I want to get the /restapi/swagger.json or /restapi/swagger.yaml URL support for the API descriptor by default. So I can use the URL in swagger UI directly for developer's convenience.

Ref) https://www.onehippo.org/labs/integrating-swagger-with-hippo-cms-restful-resources.html