[BNPL] Add feature flag for loading allowlist after syncing

This CL adds the feature flag for loading BNPL allowlist after BNPL
issuers are loaded.

This flag will be used as a kill-switch flag, so we are not exposing it
to the chrome://flags UI.

Bug: 356443046, b:436358918
Change-Id: Ifc37b79c48f2f3461d7d7743ece3171e5ce1d281
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6820698
Reviewed-by: Vinny Persky <vinnypersky@google.com>
Commit-Queue: Yishui Liu <yishuil@google.com>
Cr-Commit-Position: refs/heads/main@{#1496666}