integrate connectivitycheck.grapheneos.org as an option for connectivity / captive portal checks - MUST preserve the option to mimic the standard checks to blend in

[thestinger]

There are already settings for this, so it just needs to be exposed in the Settings app as an option. To start, it would make sense to list "GrapheneOS" and "Standard" as the options. It would be fine to make "GrapheneOS" into the default once this is available as an option, and it's one more thing for people to change to hide that they're running GrapheneOS. These changes are already all documented in the default connections section so it's just one more thing for people to change which isn't too bad. The important thing is preserving the ability to use the standard URLs.

The trivial service is already implemented:

% curl -w '%{http_code}\n' http://connectivitycheck.grapheneos.org/generate_204 
204

% curl -w '%{http_code}\n' https://connectivitycheck.grapheneos.org/generate_204
204

As far I know, the checks in Vanadium (Chromium) are active too, so those probably need to be adjusted to respect the system setting. It would be less than ideal to have separate settings.

[thestinger]

There are 3 settings: http URL, https URL and fallback URL. For now, the http URL could probably be reused as the fallback URL, or we could make another DNS entry for it.