feat: add stateless UDP forwarding mode (#853)

Add stateless=true metadata option to UDP listener/handler for raw
datagram forwarding without per-client session tracking, similar to
NGINX Stream Module behavior.

- udp.NewListener: add Stateless field to ListenConfig; branch in
  NewListener/Accept to skip connPool/listenLoop when stateless
- datagramConn: lightweight net.Conn+PacketConn wrapping a single
  UDP datagram — no channels, mutexes, or pool logic
- forward handler: add handleRawDatagram for single request-response
  cycle; extract dialTarget to share hop selection/dial/proxyproto
  preamble with handleRawForwarding

Usage: gost -L "udp://:10000/127.0.0.1:2000?stateless=true"

handler/forward/local/handler.go

@@ -206,6 +206,10 @@ func (h *forwardHandler) Handle(ctx context.Context, conn net.Conn, opts ...hand
 		return errRouterNotAvailable
 	}
 
+	if h.md.stateless {
+		return h.handleRawDatagram(ctx, conn, ro, log, network, "udp")
+	}
+
 	var proto string
 	if network == "tcp" && h.md.sniffing {
 		if h.md.sniffingTimeout > 0 {