Provide at least one reference implementation for v15

[rnestler]

Since we encountered annoying issues in v14 while implementing it in spaceapi-rs for example I think we should start implementing it at least once before the release.

• v15: Add linked_spaces, country_code, hint and areas spaceapi-community/spaceapi-rs#118

[rnestler]

There is a generated reference implementation in go available here: https://github.com/spaceapi-community/go-spaceapi-spec