Remove @domain.com from UPN during authentication

[amorrowbellarmine]

This is part of an alternative solution to the request for using the sAMAccountname from AD.
#2157
#2212

This pull requests add an option to the authentication plugin to remove the @domain.com portion of a person's username found in the claim information. The new "stripped" username is then used to match a corresponding Moodle user. This is useful for organizations that need the Moodle username to remain in the sAMAccountname format for 3rd party integrations.

The usersync code of the o365 plugin already has a code snippet where it splits the user's UPN for matching it to a Moodle username. This request borrows that code and adds it to the OIDC authentication plugin.

o365-moodle/local/o365/classes/feature/usersync/main.php

Lines 991 to 995 in 254fce2

	$upnsplit = explode('@', $upnlower);
	if (!empty($upnsplit[0])) {
	$aadusers[$i]['upnsplit0'] = $upnsplit[0];
	$usernames[] = $upnsplit[0];
	}