fetch extrinsic origin metadata from GitHub

We should periodically archive the metadata that GitHub associated to each repository.

At the time of writing these info are retrievable via the repos/ API endpoint, see:

$ curl --silent -H "Accept: application/vnd.github.v3+json" https://api.github.com/repos/octocat/hello-world
{
  "id": 1296269,
  "node_id": "MDEwOlJlcG9zaXRvcnkxMjk2MjY5",
  "name": "Hello-World",
  "full_name": "octocat/Hello-World",
  "private": false,
  "owner": {
    "login": "octocat",
[...]
  "deployments_url": "https://api.github.com/repos/octocat/Hello-World/deployments",
  "created_at": "2011-01-26T19:01:12Z",
  "updated_at": "2020-10-12T19:39:07Z",
  "pushed_at": "2020-10-10T17:46:53Z",
  "git_url": "git://github.com/octocat/Hello-World.git",
[...]
  "temp_clone_token": null,
  "network_count": 1468,
  "subscribers_count": 1689
}

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Migrated from T1740 (view on Phabricator)