GitLab: add support for the nodeinfo well-known URI

@gracinet bonjour!

I’m going to try and implement this simple feature. It is essentially a matter of adding an endpoint. The .well-known path is already reserved for webfinger based on the webfinger gem and the associated tests can be used for inspiration.

Since this is my first time trying to implement anything in the GitLab codebase, I would welcome any advice you may have :slight_smile:

Very very rough draft.