rcaps: Introduce RegionalCapabilitiesService::IsInEeaCountry

Move the source of truth for the IsEeaChoiceCountry checks to the
regional_capabilities component, and add a service method to check
the EEA presence. We will migrate away from relying on the
search_engines component for this in a follow-up.

Also moves the `--search-engine-choice-country` switch to the
regional_capabilities component (where it is processed). The header
is temporarily forwarded, includes will be fully updated in
follow-up CLs.

Bug: 328040066
Change-Id: Ic899c43afb3cb0897fb65d5058ca92b30397bbdb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6177035
Commit-Queue: Nicolas Dossou-Gbété <dgn@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1408650}