feat: add Windows UDP broadcast relay (#2222)

This may helps games to find rooms in virtual network.

- add opt-in Windows UDP broadcast relay config flag and CLI/env plumbing
- capture local UDP broadcasts with Windows raw sockets, normalize packets, and inject them via PeerManager

easytier/src/common/config.rs

@@ -72,6 +72,7 @@ pub fn gen_default_flags() -> Flags {
         instance_recv_bps_limit: u64::MAX,
         disable_upnp: false,
         disable_relay_data: false,
+        enable_udp_broadcast_relay: false,
     }
 }