[ntp-composebox] Add support for thumbnail previews to the UI.

This CL adds support for showing the snapshots in the UI behind a feature param. Another future CL will update the snapshot rendering on high-DPI displays.

Demo: https://screencast.googleplex.com/cast/NTA5Mzg3ODc1MDI0ODk2MHwzNDNiY2FiMS03Mg

Bug: 443939809
Change-Id: Ief5e0b0c8629226a4ce4c16b698d0d4bde8abde7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6965307
Reviewed-by: Nihar Majmudar <niharm@google.com>
Auto-Submit: Aviv Kiss <avivkiss@google.com>
Commit-Queue: Aviv Kiss <avivkiss@google.com>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1522486}