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"

listener/udp/listener.go

@@ -76,6 +76,7 @@ func (l *udpListener) Init(md md.Metadata) (err error) {
 		ReadBufferSize: l.md.readBufferSize,
 		Keepalive:      l.md.keepalive,
 		TTL:            l.md.ttl,
+		Stateless:      l.md.stateless,
 		Logger:         l.logger,
 	})
 	return

listener/udp/metadata.go

@@ -20,6 +20,7 @@ type metadata struct {
 	backlog        int
 	keepalive      bool
 	ttl            time.Duration
+	stateless      bool
 }
 
 func (l *udpListener) parseMetadata(md mdata.Metadata) (err error) {
@@ -43,5 +44,7 @@ func (l *udpListener) parseMetadata(md mdata.Metadata) (err error) {
 		l.md.readQueueSize = defaultReadQueueSize
 	}
 
+	l.md.stateless = mdutil.GetBool(md, "stateless")
+
 	return
 }