MINOR: ssl: add ssl_{c,s}_chain_der fetch methods

MINOR: ssl: add ssl_{c,s}_chain_der fetch methods

Following work from Arjen and Mathilde, it adds ssl_{c,s}_chain_der
methods; it returns DER encoded certs from SSL_get_peer_cert_chain

Also update existing vtc tests to add random intermediate certificates

When getting the result through this header:
  http-response add-header x-ssl-chain-der %[ssl_c_chain_der,hex]
One can parse it with any lib accepting ASN.1 DER data, such as in go:
  bin, err := encoding/hex.DecodeString(cert)
  certs_parsed, err := x509.ParseCertificates(bin)

Cc: Arjen Nienhuis <arjen@zorgdoc.nl>
Signed-off-by: Mathilde Gilles <m.gilles@criteo.com>
Signed-off-by: William Dauchy <w.dauchy@criteo.com>