Added Google Drive storage location

lib/services/drive_oauth_config.dart

@@ -0,0 +1,20 @@
+/// Fill these in once the corresponding OAuth clients exist in Google Cloud
+/// Console (see README.md "Manual setup required"). Both are needed even
+/// though only one app runs on each platform:
+///
+/// - Android sign-in (Credential Manager-based, as of google_sign_in v7)
+///   authenticates using a *Web application* type OAuth client's ID, not the
+///   Android client's own ID. The separate Android OAuth client (registered
+///   with the package name + debug/release SHA-1) is still required, but
+///   only to let Credential Manager verify this specific signed app — its
+///   client ID itself is never referenced here.
+/// - iOS uses its own iOS-type OAuth client ID directly.
+class DriveOAuthConfig {
+  /// The Web application OAuth client ID. Required for sign-in to work on
+  /// Android.
+  static const String? androidServerClientId = null; // TODO: fill in
+
+  /// The iOS OAuth client ID. Leave null if GIDClientID is instead set
+  /// directly in ios/Runner/Info.plist.
+  static const String? iosClientId = null; // TODO: fill in
+}