Add extension metadata for tagging which specification(s) the extension pertains to

[DavidBiesack]

The metadata used in http://api.specificationtoolbox.com/extensions/add/ does not specify which specification (OpenAPI 2.x, OpenAPI 3.x, AsyncAPI, etc.) the extension pertains to.

I don't know if any extension apply across different specifications, but we probably should account for that. (I.e. SLA extensions might apply to OAS and AsyncAPI).

I know there is a plan to adopt Semoasa but it is limited to only tagging an extension for oas2 or oas3