Implement the card unmask challenge acceptance button on iOS.

Before tapping the button (labeled "Send", "Continue", etc. depending on
the chosen challenge option) did nothing. Now the authentication method
specific request is sent to the payments server.

Once authentication specific views are implemented, they would be shown
when the payments server response is received.

Bug: 40282545, b/324888046
Change-Id: I012fe8f1e6e568713a631ec7532312b99508e08f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5387623


Reviewed-by: Tommy Martino <tmartino@chromium.org>
Commit-Queue: Slobodan Pejic <slobodan@chromium.org>
Reviewed-by: Siyu An <siyua@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282323}