SAML Provider: Special case for form_post when flow is skipped

Currently in the OIDC provider we can skip most of the flow when it isn't required since the final outcome is always a redirect. In SAML this is sometimes a case, but frequently we need a form post at the end to the destination ACS URL.

This causes issues with SAML-heavy installs especially with many tabs as state can still override each other

ref https://github.com/goauthentik/internal-customer-ref/issues/2