rich ntt: build updated crx packages

[tackley]

See https://github.com/brave/ads-serve/issues/4508

Currently the logic at https://github.com/brave/brave-core-crx-packager/blob/master/scripts/ntp-sponsored-images/generate.js#L34 introspects the photo.json file in order to identify the assets that need to be included in ntp-si crx packages.

This needs updating to obtain the full list of assets for a rich ntt. Perhaps ntp-si-assets should simply publish an additional assets.json or assets.txt that lists the assets to be included, avoiding the need for this code to have awareness of the internal layout of the json files used to configure new tab takeovers.