Toolbox OpenAPI 3 support PHASE 1

We're aiming to support OAS 3 at the same time than system, but we could potentially introduce support for OAS 3 with toolbox without any changes in system for service configuration, the only limitation would be updating the Active Docs.

// dev notes

OAS for configuration should work at the backend level

Potentially OAS for configuration could work at both the Product and Backend level in order to keep backward compatibility with older versions of 3scale or for those customers who wish to declare all at the product level. Which is possible except for the private API URL. System API takes care of creating backend with private URL in that case.