Spring Security Add HttpMessageConverter that constructs a RelyingPartyRegistration

It would be nice to have a simple way to convert an <IDPSSODescriptor> into a RelyingPartyRegistration. HttpMessageConverter seems to be a good interface to achieve this, as it could then be used with an application's RestTemplate definition:

RestOperations rest = new RestTemplate(Arrays.asList(
    new OpenSamlRelyingPartyRegistrationBuilderHttpMessageConverter()));
rest.getForObject(metadataLocation, RelyingPartyRegistration.Builder.class)

This would also be a nice step towards simplifying #8484.