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>
diff --git a/reg-tests/ssl/common.pem b/reg-tests/ssl/common.pem
index ca8778e..206e417 100644
--- a/reg-tests/ssl/common.pem
+++ b/reg-tests/ssl/common.pem
@@ -62,4 +62,56 @@
 vsAP/ZoS1CBirJfm3i+Gshh+VeH+TAmO/NBBYCfzBdgkNz4tJCkOc7CUT/NQTR/L
 N2OskR/Fkge149RJi7hHvE3gk/mtGtNmHJPuQ+s=
 -----END CERTIFICATE-----
-
+-----BEGIN CERTIFICATE-----
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