signInWithCustomToken function
database rules key off, so a rule can say "this device" rather than "anyone signed in".
Implementation
Future<AuthOutcome> signInWithCustomToken(String token) {
final t = token.toNativeUtf8();
return _awaitSignIn(
(port) => fdbAuthSignInWithCustomToken(t.cast(), port),
).whenComplete(() => calloc.free(t));
}