Add support for Shibboleth v4 CSRF token
Add support for Shibboleth v4 CSRF token

Shibboleth IdP v4 introduced a CSRF token as part of the login flow.
This commit extracts the token and includes it in subsequent responses.
Without this the login flow won't complete and saml2aws panics.

Fixes #638

Versent/saml2aws,Versent/saml2aws