Google Cloud OAuth settings changed; clasp login no longer works

[akhalilx]

Google Cloud has changed it's OAuth settings, moving it from:

APIs & Services > OAuth consent screen

To:

Google Auth Platform -> Audience

Furthermore, the way scopes are granted and callbacks are configured have completely changed. Therefore the current clasp login instructions are incorrect and clasp login no longer works, returning:

No access, refresh token, API key or refresh handler callback is set.

The only login method that still works is an automatically generated global for Apps Script.