[CILogon] Make CILogon's username_claim callable for consistency across authenticators

[consideRatio]

Before #717 GenericOAuthenticator was the odd duck allowing username_claim to be callable, but after that PR CILogonOAuthenticator becomes the odd dock by not allowing its username_claim implementation to be callable.

For consistency, we should make it callable, and by doing that resolve #712 by doing so.