[BNPL][Clank] Create BnplUiDelegate skeleton and implementations

This CL introduces BnplUiDelegate to handle platform-specific UI for the
Buy Now, Pay Later (BNPL) flow. This delegate is implemented by
platform-specific classes (DesktopBnplUiDelegate and
AndroidBnplUiDelegate). This abstracts away UI logic, allowing the core
BNPL flow in BnplManager to remain platform-agnostic.

This is part of the effort to expand the Chrome Desktop BNPL feature to
Android. The goal is to reuse as much of the existing BnplManager as
possible while avoiding platform-specific build flags.

Design: go/autofill-clank-bnpl

Low-Coverage-Reason: TRIVIAL_CHANGE This CL only adds a skeleton class
Bug: 430575808, 438783909
Change-Id: I0a6df704fbea2b296946c6b93deb7bf27b0213d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6825669
Reviewed-by: Vinny Persky <vinnypersky@google.com>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Fernando Ramirez <ferny@google.com>
Cr-Commit-Position: refs/heads/main@{#1503406}