Add a todo WRT correctly setting the X-Goog-AuthUser header

See flutter/flutter#80905

packages/google_sign_in/google_sign_in/lib/google_sign_in.dart

@@ -108,6 +108,8 @@ class GoogleSignInAccount implements GoogleIdentity {
     final String? token = (await authentication).accessToken;
     return <String, String>{
       "Authorization": "Bearer $token",
+      // TODO(kevmoo): Use the correct value once it's available from authentication
+      // See https://github.com/flutter/flutter/issues/80905
       "X-Goog-AuthUser": "0",
     };
   }