feat: regional country codes (#2567)

Co-authored-by: vkamn <vk@amnezia.org>

client/core/utils/api/apiUtils.h

@@ -25,6 +25,9 @@ namespace apiUtils
 
     QString getPremiumV1VpnKey(const QJsonObject &serverConfigObject);
     QString getPremiumV2VpnKey(const QJsonObject &serverConfigObject);
+
+    // ISO2-style segment for flagKit assets (e.g. US-WEST -> US). Do not use in API request bodies.
+    QString countryCodeBaseForFlag(const QString &fullCountryCode);
 }
 
 #endif // APIUTILS_H