[Watermark 2.0] Add kEnableWatermarkCustomization feature flag

This change introduces the base::Feature flag
kEnableWatermarkCustomization which will be used to control
the availability of watermark style customization options.

Currently, this flag is not connected to any logic; that will be
done in a follow-up CL. This is part of the effort to allow rolling
out the watermark customization (Watermark 2.0) features gradually.

Bug: 425991737
Change-Id: I7202634883fc476a2f58dc8a809597655d8fb258
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6652639
Reviewed-by: Nasser Al-shawwa <alshawwa@chromium.org>
Reviewed-by: Dominique Fauteux-Chapleau <domfc@chromium.org>
Commit-Queue: Adam Kassi Lahlou <adamkl@google.com>
Cr-Commit-Position: refs/heads/main@{#1476208}