[Card Benefits][Curinos] Update card benefit eligibility check

As part of scaling card benefits with Curinos, we will begin syncing
benefit_source to identify the source of the card benefits (either a
direct issuer or Curinos). We will begin using benefit_source instead of
issuer_id to check if a card is eligible for card benefits.

This CL adds checks to see if a card is eligible for flat rate benefits
sourced from Curinos.

Note: the transition from using benefit_source instead of issuer_id will
be flag guarded (kill-switch) by kAutofillEnableCardBenefitsSourceSync.
This flag is disabled by default and will be enabled by default once the
server begins sending benefit_source 100% of the time.

go/pay-autofill-chrome-scaling-benefits-design

Bug: 404870102
Change-Id: I720dac27431473e2d7e427c30baf59fccbd6818f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6469358
Commit-Queue: Fernando Ramirez <ferny@google.com>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Reviewed-by: Vinny Persky <vinnypersky@google.com>
Reviewed-by: Siyu An <siyua@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1460426}