[CustomizationV2] Add show_promo parameter to kNewTabPageCustomizationV2

This CL adds a new boolean parameter `show_promo` to the
`kNewTabPageCustomizationV2` feature flag in the segmentation platform.
The parameter defaults to false.

Bug: 423579377
Change-Id: Ib3e59c6d90d2b562e8a9f4b63993b8cdf1be52dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7830527
Reviewed-by: Siddhartha S <ssid@chromium.org>
Commit-Queue: Xinyi Ji <xinyiji@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1627703}
NOKEYCHECK=True
GitOrigin-RevId: 31bd5962829e20569da81731e0634ce7fc33f319