[WIP] MINOR: Use named class for ExpiringCredential to improve `principalLogText()` output

[ijuma]

ExpiringCredentialRefreshingLogin.principalLogText() outputs the expiring credential simple class name, which is empty for anonymous classes.

WIP: I also noticed some weirdness in the principalLogText, need to figure out the details of that before this is ready for review.

Committer Checklist (excluded from commit message)

• [ ] Verify design and implementation
• [ ] Verify test coverage and CI build status
• [ ] Verify documentation (including upgrade notes)