This implements the AuthManager class and its needed interfaces and
subclasses, and integrates them into the backend portion of MediaWiki.
Integration with frontend portions of MediaWiki (e.g. ApiLogin,
Special:Login) is left for a followup.
Bug: T91699
Bug: T71589
Bug: T111299
Co-Authored-By: Gergő Tisza <gtisza@wikimedia.org>
Change-Id: If89d24838e326fe25fe867d02181eebcfbb0e196
