MINOR: ssl: add ssl_sock_get_pkey_algo function

ssl_sock_get_pkey_algo can be used to report pkey algorithm to log
and ppv2 (RSA2048, EC256,...).
Extract pkey information is not free in ssl api (lock/alloc/free):
haproxy can use the pkey information computed in load_certificate.
Store and use this information in a SSL ex_data when available,
compute it if not (SSL multicert bundled and generated cert).
diff --git a/include/proto/ssl_sock.h b/include/proto/ssl_sock.h
index ef03de6..91456e2 100644
--- a/include/proto/ssl_sock.h
+++ b/include/proto/ssl_sock.h
@@ -56,6 +56,7 @@
 int ssl_sock_get_cert_used_sess(struct connection *conn);
 int ssl_sock_get_cert_used_conn(struct connection *conn);
 int ssl_sock_get_remote_common_name(struct connection *conn, struct chunk *out);
+int ssl_sock_get_pkey_algo(struct connection *conn, struct chunk *out);
 unsigned int ssl_sock_get_verify_result(struct connection *conn);
 #if (defined SSL_CTRL_SET_TLSEXT_STATUS_REQ_CB && !defined OPENSSL_NO_OCSP)
 int ssl_sock_update_ocsp_response(struct chunk *ocsp_response, char **err);