option.WithAuthCredentials is broken

`option.WithAuthCredentials` is presently (v4.18.0) broken in 2 places:

1. "google.golang.org/api@v0.231.0" will not properly switch to using the "new auth lib". A newer version of google API library is required, such as the recent "google.golang.org/api@v0.249.0".
2. `option.WithScopes`, used at present by the implementation will have no effect when `option.WithAuthCredentials` is specified (assuming p.1). The scopes must be given directly to the constructor of the whatever `auth.Credentials` object needed; at least this is the case with `externalaccount.NewCredentials`.
3. Because of the above, I'd say `internal.FirebaseScopes` shall be exported from externally visible package. Those scopes are already publicly advertised API feature, not some sort of internal detail (https://developers.google.com/identity/protocols/oauth2/scopes).