Cache sponsored tile favicons for use elsewhere in the app

[data-sync-user]

Currently, we cache the bundled DefaultSuggestedSites + Google tile favicon after first use so subsequent visits to these sites will also use our high quality icon.

It appears that maybe the SponsoredTiles are not using the same logic, as per this bug: https://github.com/mozilla-mobile/firefox-ios/issues/22098 and this bug: https://github.com/mozilla-mobile/firefox-ios/issues/22109

However the issue may be in hero images or the overlay. Some investigation needs to be done, and maybe an optimization can be made so these sponsored websites have consistent good treatment.

┆Issue is synchronized with this Jira Task