Added support for Association Online sites to behave as OAuth2 servers.

Essentially added extra cases to switch statements which switched on the
provider type, to cater for "ao" provider type.
Only request "basic" details, not all user details.

First/last name + email should be enough to populate a relevant Moodle
user, and this way, we don't alarm the users by requesting access to
everything AO knows about them.
Removed providers which are not Association Online.

This plugin now only supports Associaiton Online, because it will make
configuration easier for devs who need it. Also, it is unlikely that people
who are uninterested in AO would use this fork, as it doesn't add any
benefit over the upstream version in such a case.