Feature: Support using different claim than email address as username

It was requested in https://github.com/canonical/authd/issues/574#issuecomment-4044380917 to support using a different claim than the email address as the username:

> Our email address is on the format [firstname.middlename.lastname@domain.com](mailto:firstname.middlename.lastname@domain.com), which makes at least for some of us the username in ubuntu very long, even if we remove the domain part.
>
> It would be nice if we could map our actual username in keycloak to a claim and send it to authd so we get the same username in ubuntu as we use in the rest of the organization.

No files, tests, or entry points are named. Read the linked request and issue discussion first, then identify how the requested claim should be configured and what behavior is expected. Done means authd can use a non-email claim so the Ubuntu username matches the organization's username.

Written by the indexing model from the issue text.