Implement base endpoint of the new API

Context

We are now rolling out a new version of the Container Registry in GitLab.com that includes a metadata database (&5523 (closed)). Among others, this database will allow for efficient storage usage calculations.

This issue is part of a work plan to expose the deduplicated size of image repositories in the GitLab UI and API (&7575 (closed)).

Goal

Implement the base endpoint (GET /gitlab/v1) defined in #516 (closed).