Allow overridden user agent in auction update url requests.

This update improves the fidelity of our auction requests by incorporating user agent overrides.

This is behind a feature flag that is off by default.

Change-Id: I13b2b6a14b4d92619edcdf2ca5bb2e67560a119d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5949605
Reviewed-by: Caleb Raitto <caraitto@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: Youssef Bourouphael <ybourouphael@google.com>
Cr-Commit-Position: refs/heads/main@{#1372899}