Support `x-extensible-enum` OpenApi type

[sjoerdtalsma]

This was introduced by Zalando's API team and picked up by the open api community.

It is precisely what the Enumerable type is invented for.

Reference:

• https://opensource.zalando.com/restful-api-guidelines/#112
• https://github.com/OAI/OpenAPI-Specification/issues/1552
• https://swagger.io/docs/specification/data-models/enums/
• https://github.com/Azure/autorest/blob/master/docs/extensions/readme.md#x-ms-enum

[sjoerdtalsma]

Interesting read: https://github.com/zalando/restful-api-guidelines/issues/412