Add extms extension

Reviewed-by: NMatt Caswell <matt@openssl.org>

util/TLSProxy/ClientHello.pm

@@ -59,6 +59,7 @@ use parent 'TLSProxy::Message';
 
 use constant {
     EXT_ENCRYPT_THEN_MAC => 22,
+    EXT_EXTENDED_MASTER_SECRET => 23,
     EXT_SESSION_TICKET => 35
 };