Saml2WebSsoAuthenticationRequesFilter should be post-processed

[jzheaux]

It should be possible to use an ObjectPostProcessor to post-process the configuration of Saml2WebSsoAuthenticationRequestFilter.

Otherwise, there is no clear way to configure its RequestMatcher or its Saml2AuthenticationRequestContextResolver other than to bypass saml2Login on the DSL and instantiate the filter itself.

For additional context, note that OAuth2LoginConfigurer post-processes its equivalent filter, OAuth2AuthorizationRequestRedirectFilter.