Design the authentication API for Cristal + implementation for the XWiki backend

Note: authentication is delegated to the backend (e.g., if XWiki is configured to use OIDC, then Cristal will use that).

• https://forum.xwiki.org/t/authentication-design/15649/3
• https://design.xwiki.org/xwiki/bin/view/Proposal/Loginandregistrationflows
• https://forum.xwiki.org/t/log-in-and-registration-flows/14302