[iOS] Add password notes to CPE

This CL introduces notes for passwords features in CPE with following
changes:
* credential.h is extended with new `note` field
* password_note_cell.h adds implementation of the view for displaying
and editing notes
* credential_details_view_controller.mm is extended with a new row to
display note
* new_password_view_controller.mm is extended with new section for
notes

All changes are protected by a feature flag disabled by default.

Mocks: http://docs/presentation/d/1pYvziCIUJJtgphgPDW5NadgSbeSinH4zQVWPVgrYkys?resourcekey=0-ikKcsq0JtetENgJP-7JZYw#slide=id.g10f9d40d237_0_0
Screencast: https://drive.google.com/file/d/1hi7WLAJRz5REYDt_cXCJu4wfbArSaT3s

Bug: 1414897
Change-Id: Ia9defb2719d3b4e0ab3d2f7b023b4cb657e07c1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4307417
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Rafał Godlewski <rgod@google.com>
Reviewed-by: David Jean <djean@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118004}