Find directFromSellerSignalsHeaderAdSlot during auctions

This CL finds the directFromSellerSignalsHeaderAdSlot passed from
promise resolution while interest groups are being loaded, if such
signals exist.

Actually passing the retrieved signals to worklet functions will come in
a later CL.

Bug: 1462720
Change-Id: I8ca32a1f36833559f7b382e804898b1c3a0e38e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4716101


Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1180457}