Only validate certificate and set client certificate tp authn request.

Made it possible for the IdP to only validate the certificate without
verifying the signature.
This is needed when the proxy sends the SP certificate to the IdP.

Made it possible to send the certificate that should be used during the
creating of the authn request.

want_authn_requests_only_with_valid_cert overrides want_authn_requests_signed. If want_authn_requests_only_with_valid_cert no signature will be validated. But if want_authn_requests_only_with_valid_cert is false and want_authn_requests_signed is true, cert and signature is always validated.