[gateway] Access API version number in Expression language

[vdusauto]

It would be a good enhancement to be able to access metadata of an API like its version (defined in the portal) during calls made to the gateway, via in expression language for example.

This could be used to return in response headers or in a /version subpath to provide insight to consumers of the API.

This could be accessible from within a MOCK or Groovy policy even using a syntax like {#api.version} or api.version.