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

This CL is the fourth 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 implements processing of the captured image frames. VisionKit's
OCR ability is used to extract text, which is then processed looking for
card numbers and expiry dates (in the form of mm/yy). To mitigate
misrecognition of numbers various strategies are used:

1. A top-N algorithm that captures multiple frames and then selects
   the most commonly recognized card number and expiry dates.
2. A substitution algorithm that looks for commonly mistaken
   letters/numbers (e.g. 'B' for 8) and swaps to the number.
3. Minimizing the chance of a bad card number being recognized by
   luhn checking the extracted number before accepting it.

Further enhancements are being considered, but this CL completes the
basic prototype (behind a flag) that can now be tested by others.

Bug: 435324025
Change-Id: I9464bf3073cb57d05b5e97508934f1a4b3c53890
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6821169
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1511707}