Access to cached authentication object

In our apps we call the method `restorePreviousSignInWithCallback:` every startup. This is recommended by the [documentation](https://developers.google.com/identity/sign-in/ios/sign-in#2_attempt_to_restore_the_users_sign-in_state), and we do this in order to be informed that the user's token was revoked.
`restorePreviousSignInWithCallback:` can fail for many reasons, for example if there was a failure to refresh the token. In most of these cases, we would still like to get the cached token, even if it expired or about to expire. However currently there is no way to get the token if an error occurred during `restorePreviousSignInWithCallback:`.

One solution could be to make [restorePreviousSignInNoRefresh](https://github.com/google/GoogleSignIn-iOS/blob/c90a734cbfbbf2b1870a06407b2ef59929357ac9/GoogleSignIn/Sources/GIDSignIn_Private.h#L38) public in some form.