OAuth2 review and improve

OAuth2 has become the de facto authorisation and authentication standard for web applications.
We want to improve the support for OAuth2 in Moodle LMS. This will better allow organisations to use their identity providers and other OAuth2 service providers with Moodle LMS.

This epic will collate existing and new OAuth2 related trackers. These will be then be reviewed, a plan of work will then be devised and executed. It's likely this work will be split of several phases (with their corresponding epics linked to the parent IDEA)