This updates `gate-saml` to use the built-in SAML support in Spring
Security which uses OpenSAML 4.x. Nearly all the previously supported
properties for SAML are still supported, though a couple niche options
no longer seem to be configurable.

This also introduces `AuthenticationService`, a variation of
`PermissionService` which can also return a user's granted authorities
in one login call. It was also used for exception translation previously
as retrofit exceptions are not serializable which would cause errors in
Spring Security authentication failure error handlers, but the
underlying exception being thrown has since been updated to avoid that
problem.