Rename X-fledge-bidding-signals-format-version

Rename X-fledge-bidding-signals-format-version to
Ad-Auction-Bidding-Signals-Format-Version

This change add support for the new name, but continues supporting the
old name, for compatibility. Renaming usages of the old header in tests
and comments will come in a follow-up CL.

Bug: 1440204
Change-Id: Ie28feaee295a189e16bf5c54a10481ae47831a78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4558704


Reviewed-by: Russ Hamilton <behamilton@google.com>
Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148184}