Saml2LoginConfigurer should pick up Saml2AuthenticationTokenConverter bean

[jzheaux]

Related to #10243

To customize how a RelyingPartyRegistration is resolved from a login request, an application needs to configure an Saml2AuthenticationTokenConverter in the DSL:

@Bean
SecurityFilterChain appSecurity(HttpSecurity http, RelyingPartyRegistrationRepository registrations) throws Exception {
    RelyingPartyRegistrationResolver registrationResolver = new DefaultRelyingPartyRegistrationResolver(registrations);
    Saml2AuthenticationTokenConverter authenticationConverter = new Saml2AuthenticationTokenConverter(registrationResolver);
    http
        .authorizeHttpRequests((authorize) -> authorize
            .anyRequest().authenticated()
        )
        .saml2Login((saml2) -> saml2
            .authenticationConverter(authenticationConverter)
        );

    return http.build();
}

Because a Saml2AuthenticationTokenConverter is often based on a RelyingPartyRegistrationRepository bean, this could be modestly simplified by Saml2LoginConfigurer picking up a Saml2AuthenticationTokenConverter bean:

@Bean 
Saml2AuthenticationTokenConverter authenticationConverter(RelyingPartyRegistrationRepository registrations) {
    RelyingPartyRegistrationResolver registrationResolver = new DefaultRelyingPartyRegistrationResolver(registrations);
    return new Saml2AuthenticationTokenConverter(registrationResolver);
}