fetch extrinsic origin metadata from GitLab instances

(analogous to #1740 (closed), but for repos hosted on GitLab instances)

Info about an individual repository (called "project" in GitLab terminology) can be obtained using the /projects endpoint, e.g.:

$ curl --silent "https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab" | jq     
{
  "id": 278964,
  "description": "GitLab is an open source end-to-end software development platform with built-in version control, issue tracking, code review, CI/CD, and more. Self-host GitLab on your own servers, in a container, or on a cloud provider.",
  "name": "GitLab",
  "name_with_namespace": "GitLab.org / GitLab",
  "path": "gitlab",
  "path_with_namespace": "gitlab-org/gitlab",
  "created_at": "2015-05-20T10:47:11.949Z",
  "default_branch": "master",
  "tag_list": [],
  "ssh_url_to_repo": "git@gitlab.com:gitlab-org/gitlab.git",
  "http_url_to_repo": "https://gitlab.com/gitlab-org/gitlab.git",
  "web_url": "https://gitlab.com/gitlab-org/gitlab",
  "readme_url": "https://gitlab.com/gitlab-org/gitlab/-/blob/master/README.md",
  "avatar_url": "https://assets.gitlab-static.net/uploads/-/system/project/avatar/278964/logo-extra-whitespace.png",
  "forks_count": 2523,
  "star_count": 2188,
  "last_activity_at": "2020-10-13T07:46:13.144Z",
  "namespace": {
    "id": 9970,
    "name": "GitLab.org",
    "path": "gitlab-org",
    "kind": "group",
    "full_path": "gitlab-org",
    "parent_id": null,
    "avatar_url": "/uploads/-/system/group/avatar/9970/logo-extra-whitespace.png",
    "web_url": "https://gitlab.com/groups/gitlab-org"
  }
}

---

Migrated from T2693 (view on Phabricator)