Cedar: Add ProtoOptionString() in PROTO_IMPL, to generate default option values

The WireGuard implementation will have two options that should not have a fixed default value, because they represent two keys (one is preshared, the other is private).

Instead of handling these two options differently in ProtoNewContainer(), this commit adds a new function to PROTO_IMPL: ProtoOptionString().

ProtoOptionString() takes the option's name as argument and returns a heap-allocated string that will be used as value. The function returns NULL when the option doesn't need a randomized value.

src/Cedar/Proto_OpenVPN.c

@@ -20,6 +20,7 @@ const PROTO_IMPL *OvsGetProtoImpl()
 	{
 		OvsName,
 		OvsOptions,
+		NULL,
 		OvsInit,
 		OvsFree,
 		OvsIsPacketForMe,