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)

connector/masque/metadata.go

@@ -0,0 +1,17 @@
+package masque
+
+import (
+	"time"
+
+	mdata "github.com/go-gost/core/metadata"
+	mdutil "github.com/go-gost/x/metadata/util"
+)
+
+type metadata struct {
+	connectTimeout time.Duration
+}
+
+func (c *masqueConnector) parseMetadata(md mdata.Metadata) (err error) {
+	c.md.connectTimeout = mdutil.GetDuration(md, "timeout", "connectTimeout")
+	return nil
+}