This commit separates the authentication principal, the assertion details,
and the relying party tenant into separate components. This allows the
principal to be completely decoupled from how Spring Security triggers and
processes SLO.
Specifically, it adds Saml2AssertionAuthentication, a new authentication
implementation that allows an Object principal and a Saml2ResponseAssertionAccessor
credential. It also moves the relying party registration id from
Saml2AuthenticatedPrincipal to Saml2AssertionAuthentication.
As such, Saml2AuthenticatedPrincipal is now deprecated in favor of
placing its assertion components in Saml2ResponseAssertionAccessor and
the relying party registration id in Saml2AssertionAuthentication.
Closes gh-10820
