Generate metadata (country) on the server side

[laurentsenta]

Support to #442, we want to generate most of the gateway checker dashboard on the server side.

The conformance test suite will be in charge of generating status, but the dashboard needs some metadata that are not generated by the conformance test suite:

We only care about countries for now, we'll:

• takes a json list of gateway addresses on stdin,
• output a mapping { address: { country }, address2: { country }}
• call this script in the CI, at the end of conformance generation and use it to augment the metadata field (see interop in #442)