Add Saml2AuthenticationRequestResolver

[jzheaux]

This PR simplifies the customization of AuthnRequest by introducing a component that can provide both the HttpServletRequest and RelyingPartyRegistration to the application:

@Bean 
public Saml2AuthenticationRequestResolver authenticationRequestResolver(RelyingPartyRegistrationResolver registrations) {
    OpenSaml4AuthenticationRequestResolver authenticationRequestResolver = 
            new OpenSaml4AuthenticationRequestResolver(registrations);
    authenticationRequestResolver.setAuthnRequestCustomizer((context) -> {
        AuthnRequest authnRequest = context.getAuthnRequest();
        HttpServletRequest request = context.getRequest();
        authnRequest.setForceAuthn(request.getParameter("force"));
    });
    return authenticationRequestResolver;
}

This component supercedes Saml2AuthenticationRequestFactory, Saml2AuthenticationRequestContextResolver, and Saml2AuthenticationRequestContext.

[jzheaux]

@Primedo, @fpagliar - would you be able to give this a review to see if it simplifies your SAML configuration?

[jzheaux]

Merged with d538423