Support for Swagger 3 (OpenAPI 3)

Provide support of the new 3.0 version of the Swagger specification (which was rebranded into OpenApi)  

Useful references:
- [Migrating to Swagger-PHP v3.x](http://zircote.com/swagger-php/Migrating-to-v3.html)
- [A Guide to What’s New in OpenAPI 3.0](https://swagger.io/blog/news/whats-new-in-openapi-3-0/)