Add MASQUE/CONNECT-UDP support (RFC 9298)

Implement UDP tunneling over HTTP/3 using HTTP Datagrams (RFC 9297):
- Add masque handler for server-side CONNECT-UDP
- Add masque connector and h3-masque dialer for client-side
- Add enableDatagrams option to HTTP/3 listener
- Add shared utilities for datagram connections and path parsing

Resource management and connection caching:
- Add deferred stream cleanup in connector on error paths
- Add IsClosed() and Close() methods to Client for proper session management
- Clean up stale cached clients in dialer before reuse
- Close underlying stream when DatagramConn is closed
- Move RequestStream opening from connector to dialer to enable dead
  connection detection and cache invalidation (follows QUIC dialer pattern)

handler/masque/metadata.go

@@ -0,0 +1,49 @@
+package masque
+
+import (
+	"time"
+
+	mdata "github.com/go-gost/core/metadata"
+	mdutil "github.com/go-gost/x/metadata/util"
+)
+
+const (
+	defaultBufferSize = 4096
+	defaultRealm      = "gost"
+)
+
+type metadata struct {
+	hash           string
+	bufferSize     int
+	authBasicRealm string
+
+	observerPeriod         time.Duration
+	observerResetTraffic   bool
+	limiterRefreshInterval time.Duration
+	limiterCleanupInterval time.Duration
+}
+
+func (h *masqueHandler) parseMetadata(md mdata.Metadata) (err error) {
+	h.md.hash = mdutil.GetString(md, "hash")
+
+	h.md.bufferSize = mdutil.GetInt(md, "bufferSize", "udp.bufferSize")
+	if h.md.bufferSize <= 0 {
+		h.md.bufferSize = defaultBufferSize
+	}
+
+	h.md.authBasicRealm = mdutil.GetString(md, "authBasicRealm")
+
+	h.md.observerPeriod = mdutil.GetDuration(md, "observePeriod", "observer.period", "observer.observePeriod")
+	if h.md.observerPeriod == 0 {
+		h.md.observerPeriod = 5 * time.Second
+	}
+	if h.md.observerPeriod < time.Second {
+		h.md.observerPeriod = time.Second
+	}
+
+	h.md.observerResetTraffic = mdutil.GetBool(md, "observer.resetTraffic")
+	h.md.limiterRefreshInterval = mdutil.GetDuration(md, "limiter.refreshInterval")
+	h.md.limiterCleanupInterval = mdutil.GetDuration(md, "limiter.cleanupInterval")
+
+	return nil
+}