client: Add `username` to `User` and start using it (#60107)

This PR adds a `username` field to the `User` and starts using it
instead of the `github_login`.

Closes CLO-952.

Release Notes:

- N/A
53e4d34a71Marshall Bowers committed on 6/29/2026, 8:21:46 PM· committed by GitHubparent29faf94
16 files changedLine totals unavailable