Basic pluto with IKEv2 using X.509 on the responder (east), and Strongswan on
the initiator (west).

This is the reverse test of interop-ikev2-strongswan-04-x509-responder

This is the basic test for Authentication method as Digital Signatures as 
specified in RFC 7427. Currently supported for RSA-PSS.

This includes a new notification of type SIGNATURE_HASH_ALGORITHMS.

Authentication is successful with rsa-sha2_384

Caveat: strongswan with authby=rsasig accepts RSA-PSS SHA2 signatures,
but responds with RSAv1.5 SHA2 signatures, which we refuse. According
to RFC 7427/8247 RSA-PSS SHA2 is MUST and RSAv1.5 SHA2 is MAY implement.
Workaround is to explicitly configure strongswan for RSA-PSS