default CompletionStage<Void> beforeInitiatingSso(ServiceRequestContext ctx, HttpRequest req,
                                                      MessageContext<AuthnRequest> message,
                                                      SamlIdentityProviderConfig idpConfig) {
        return UnmodifiableFuture.completedFuture(null);
    }
Invoked before the service provider sends an authentication request to an identity provider.

@param ctx the {@link ServiceRequestContext} of {@code req}
@param req the {@link Request} being handled
@param message the {@link MessageContext} of the {@link AuthnRequest} being sent to the identity
               provider
@param idpConfig the configuration of the identity provider that the request is sending to
saml/src/main/java/com/linecorp/armeria/server/saml/SamlSingleSignOnHandler.java