Verify Sign Blob works outside GDU

Ensure that the methods in [`SignBlobInterface`](https://github.com/googleapis/google-auth-library-php/blob/main/src/SignBlobInterface.php), [`Iam`](https://github.com/googleapis/google-auth-library-php/blob/main/src/Iam.php), and [`IamSignerTrait`](https://github.com/googleapis/google-auth-library-php/blob/main/src/IamSignerTrait.php) work outside the GDU