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)

dialer/http3/masque/metadata.go

@@ -0,0 +1,43 @@
+package masque
+
+import (
+	"time"
+
+	mdata "github.com/go-gost/core/metadata"
+	mdutil "github.com/go-gost/x/metadata/util"
+)
+
+const (
+	mdKeyHost             = "host"
+	mdKeyKeepAlive        = "keepAlive"
+	mdKeyKeepAlivePeriod  = "ttl"
+	mdKeyHandshakeTimeout = "handshakeTimeout"
+	mdKeyMaxIdleTimeout   = "maxIdleTimeout"
+	mdKeyMaxStreams       = "maxStreams"
+)
+
+type metadata struct {
+	host string
+
+	// QUIC config options
+	keepAlivePeriod  time.Duration
+	maxIdleTimeout   time.Duration
+	handshakeTimeout time.Duration
+	maxStreams       int
+}
+
+func (d *masqueDialer) parseMetadata(md mdata.Metadata) (err error) {
+	d.md.host = mdutil.GetString(md, mdKeyHost)
+
+	if mdutil.GetBool(md, mdKeyKeepAlive) {
+		d.md.keepAlivePeriod = mdutil.GetDuration(md, mdKeyKeepAlivePeriod)
+		if d.md.keepAlivePeriod <= 0 {
+			d.md.keepAlivePeriod = 10 * time.Second
+		}
+	}
+	d.md.handshakeTimeout = mdutil.GetDuration(md, mdKeyHandshakeTimeout)
+	d.md.maxIdleTimeout = mdutil.GetDuration(md, mdKeyMaxIdleTimeout)
+	d.md.maxStreams = mdutil.GetInt(md, mdKeyMaxStreams)
+
+	return nil
+}