feat(android): return nonce on sign in

[robingenz]

Is your feature request related to a problem? Please describe:

Currently, signing in to Apple using the Firebase JS SDK is not possible on Android, among other reasons (see #48), because the nonce is not returned.

Describe the solution you'd like:

Return the nonce on sign in.

OAuthCredential (see #44):

export interface OAuthCredential extends AuthCredential {
  accessToken?: string;
  idToken?: string;
  secret?: string;
  nonce?: string;
}

Describe alternatives you've considered:

No alternative options.

Additional context:

[robingenz]

I could not find a way to get the nonce value on Android.
It would be possible to manually handle the sign-in flow (see here) so that you can generate the nonce value yourself, but that would increase the setup effort significantly.