resolve method
Used to resolve the auth using the getSecret function.
Implementation
@override
ResolvedAuth resolve(String? Function(String name) getSecret) {
  final secret = getSecret(secretName);
  if (secret == null) {
    throw MissingSecretsException(this);
  }
  return ResolvedAuth(
    headers: {'Authorization': '$scheme $secret'},
    params: const {},
  );
}