[iOS] Add credit card scanner prototype (1/4)

This CL is the first in a chain of CLs that add a prototype credit card
scanner for iOS, which can be triggered when adding cards manually via
Payment Method settings. The prototype resurrects and improves upon a
previous ~2019 attempt that was removed in
https://chromium-review.googlesource.com/c/chromium/src/+/2534781

This CL adds a feature and flag for the prototype, and adds a "Use
Camera" button to the Payment Methods --> Add Card screen (when the
feature is enabled). The button currently does not do anything when
clicked.

Bug: 435324025
Change-Id: I98b5603b2c1eec009a82c1fa06eaa7a47a5d63f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6806353
Reviewed-by: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1498834}