feat(saml): Update SAML to use Spring Security

[jvz]

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.