feat(dialer): add utls dialer for TLS ClientHello fingerprint simulation

Add a new "utls" dialer type that uses the refraction-networking/utls
library to mimic browser TLS fingerprints (Chrome, Firefox, Safari, etc.).

- x/dialer/utls/dialer.go: Registered as "utls", mirrors the TLS dialer
  with Handshake() using utls.UClient() for fingerprint emulation.
- x/dialer/utls/fingerprint.go: Maps string names (chrome, firefox, ios,
  safari, edge, randomized, golang, custom) to utls.ClientHelloID presets.
- x/dialer/utls/metadata.go: Parses fingerprint from dialer metadata.
- go.mod: Add github.com/refraction-networking/utls v1.8.2.

The fingerprint is dialer-only metadata — the standard TLS dialer is
untouched. Falls back to crypto/tls if no fingerprint is configured.

Closes go-gost/gost#31

dialer/utls/dialer.go

@@ -0,0 +1,106 @@
+package utls
+
+import (
+	"context"
+	"crypto/tls"
+	"net"
+	"time"
+	"unsafe"
+
+	"github.com/go-gost/core/dialer"
+	"github.com/go-gost/core/logger"
+	md "github.com/go-gost/core/metadata"
+	xctx "github.com/go-gost/x/ctx"
+	xnet "github.com/go-gost/x/internal/net"
+	"github.com/go-gost/x/internal/net/proxyproto"
+	"github.com/go-gost/x/registry"
+	utls "github.com/refraction-networking/utls"
+)
+
+func init() {
+	registry.DialerRegistry().Register("utls", NewDialer)
+}
+
+type utlsDialer struct {
+	md      metadata
+	log     logger.Logger
+	options dialer.Options
+}
+
+func NewDialer(opts ...dialer.Option) dialer.Dialer {
+	options := dialer.Options{}
+	for _, opt := range opts {
+		opt(&options)
+	}
+
+	return &utlsDialer{
+		log:     options.Logger,
+		options: options,
+	}
+}
+
+func (d *utlsDialer) Init(md md.Metadata) (err error) {
+	return d.parseMetadata(md)
+}
+
+func (d *utlsDialer) Dial(ctx context.Context, addr string, opts ...dialer.DialOption) (net.Conn, error) {
+	var options dialer.DialOptions
+	for _, opt := range opts {
+		opt(&options)
+	}
+
+	conn, err := options.Dialer.Dial(ctx, "tcp", addr)
+	if err != nil {
+		d.log.Error(err)
+	}
+
+	if d.md.keepalive {
+		xnet.ApplyKeepalive(conn, net.KeepAliveConfig{
+			Enable:   true,
+			Idle:     d.md.keepaliveIdle,
+			Interval: d.md.keepaliveInterval,
+			Count:    d.md.keepaliveCount,
+		})
+	}
+
+	conn = proxyproto.WrapClientConn(
+		d.options.ProxyProtocol,
+		xctx.SrcAddrFromContext(ctx),
+		xctx.DstAddrFromContext(ctx),
+		conn)
+
+	return conn, err
+}
+
+// Handshake implements dialer.Handshaker.
+func (d *utlsDialer) Handshake(ctx context.Context, conn net.Conn, options ...dialer.HandshakeOption) (net.Conn, error) {
+	if d.md.handshakeTimeout > 0 {
+		conn.SetDeadline(time.Now().Add(d.md.handshakeTimeout))
+		defer conn.SetDeadline(time.Time{})
+	}
+
+	clientHelloID, ok := GetClientHelloID(d.md.fingerprint)
+	if ok {
+		d.log.Debugf("utls handshake with fingerprint: %s", d.md.fingerprint)
+		utlsCfg := (*utls.Config)(unsafe.Pointer(d.options.TLSConfig))
+		uconn := utls.UClient(conn, utlsCfg, clientHelloID)
+		if err := uconn.HandshakeContext(ctx); err != nil {
+			conn.Close()
+			return nil, err
+		}
+		return uconn, nil
+	}
+
+	// fingerprint not set, empty, "golang", or unknown: fall through to
+	// standard crypto/tls.
+	if d.md.fingerprint != "" {
+		d.log.Warnf("unknown utls fingerprint: %s, falling back to standard TLS", d.md.fingerprint)
+	}
+	tlsConn := tls.Client(conn, d.options.TLSConfig)
+	if err := tlsConn.HandshakeContext(ctx); err != nil {
+		conn.Close()
+		return nil, err
+	}
+
+	return tlsConn, nil
+}