Saml2Authentication should have registration id

[jzheaux]

Like OAuth2AuthenticationToken, SAML 2.0's authentication instance should have a reference to the corresponding RelyingPartyRegistration.

This is important during SAML 2.0 logout when processing LogoutRequests and LogoutResponses. It would also be valuable in circumstances where applications need to look up relying party configurations for the currently logged-in user.

[jzheaux]

The original impetus for this ticket is #8731 and since it has been reopened for additional research, we'll reopen this one as well, just in case changes there require changes here.