Updater: Add support for injecting signing flags via command line.

Bug: b/335182500
Change-Id: Iabfcb028113474f68ab973997a1b805dbc541771
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5602315


Commit-Queue: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Xiaoling Bao <xiaolingbao@chromium.org>
Auto-Submit: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313074}