option.WithAuthCredentials is broken

[oakad]

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).

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