WIP: Use case-insensitive lookup for Kerberos Spnego identity.

What does this MR do?

Clients can send the Kerberos username/principal in any case - the Kerberos server itself does not care. However, GitLab stores the Kerberos username/principal as extern_uid in a case-sensitive format. This change uses an iwhere statement rather than find_by so it doesn't matter how the client send the username/principal.

Are there points in the code the reviewer needs to double check?

Should we have specs for this? Where? There isn't a clear place to put specs for this helper class.

Why was this MR needed?

It was causing inability to authenticate for a customer in https://gitlab.zendesk.com/agent/tickets/82669

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together