Add saml2Metadata to the DSL

[marcusdacoregio]

In order to publish the metadata endpoint, we should do:

DefaultRelyingPartyRegistrationResolver relyingPartyRegistrationResolver =
        new DefaultRelyingPartyRegistrationResolver(this.relyingPartyRegistrationRepository);
Saml2MetadataFilter filter = new Saml2MetadataFilter(
        relyingPartyRegistrationResolver,
        new OpenSamlMetadataResolver());

http
    // ...
    .saml2Login(withDefaults())
    .addFilterBefore(filter, Saml2WebSsoAuthenticationFilter.class);

It would be nice if we have a new method in the DSL that achieve the same behavior just by doing:

http
    // ...
   .saml2Login(withDefaults())
   .saml2Metadata(withDefaults());