[client, relay-server] Feature/relay notification (#4083)

- Clients now subscribe to peer status changes.
- The server manages and maintains these subscriptions.
- Replaced raw string peer IDs with a custom peer ID type for better type safety and clarity.

client/iface/wgproxy/proxy.go

@@ -12,4 +12,5 @@ type Proxy interface {
 	Work()                      // Work start or resume the proxy
 	Pause()                     // Pause to forward the packages from remote connection to WireGuard. The opposite way still works.
 	CloseConn() error
+	SetDisconnectListener(disconnected func())
 }