Rename 'Auth' to 'Credentials' to match the gerrit package.
Add a couple more helpers, and add more comments.
Bug: dawn:1342
Change-Id: Ieb6d12d23bb71678e04ff694ab9085a8f5367437
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86525
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>