signInWithOtp method
Send a login OTP — signInWithOtp(email: …) or signInWithOtp(phone: …).
Routes to email-OTP or phone-OTP by which arg is set. Exactly one of
email / phone is required.
Implementation
Future<OtpStartResponse> signInWithOtp({String? email, String? phone}) {
if (phone != null) return phoneStart(phone);
if (email != null) return emailStart(email);
throw ArgumentError('signInWithOtp requires either email or phone');
}