feat: support disabling relay data forwarding (#2188)

- add a disable_relay_data runtime/config patch option
- reuse the existing avoid_relay_data feature flag when relay data forwarding is disabled

easytier/src/common/config.rs

@@ -71,6 +71,7 @@ pub fn gen_default_flags() -> Flags {
         need_p2p: false,
         instance_recv_bps_limit: u64::MAX,
         disable_upnp: false,
+        disable_relay_data: false,
     }
 }