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/fingerprint.go

@@ -0,0 +1,33 @@
+package utls
+
+import (
+	utls "github.com/refraction-networking/utls"
+)
+
+// fingerprintMap maps user-facing string names to uTLS ClientHelloID presets.
+// Each _Auto alias automatically tracks the latest known fingerprint for that
+// browser in the uTLS library.
+var fingerprintMap = map[string]utls.ClientHelloID{
+	"chrome":            utls.HelloChrome_Auto,
+	"firefox":           utls.HelloFirefox_Auto,
+	"ios":               utls.HelloIOS_Auto,
+	"safari":            utls.HelloSafari_Auto,
+	"edge":              utls.HelloEdge_Auto,
+	"randomized":        utls.HelloRandomized,
+	"randomized-alpn":   utls.HelloRandomizedALPN,
+	"randomized-noalpn": utls.HelloRandomizedNoALPN,
+	"golang":            utls.HelloGolang,
+	"custom":            utls.HelloCustom,
+}
+
+// GetClientHelloID returns the utls.ClientHelloID for the given fingerprint
+// name. The empty string and "golang" both return ok=false, signalling the
+// caller to fall through to standard crypto/tls. Unknown names also return
+// ok=false.
+func GetClientHelloID(name string) (utls.ClientHelloID, bool) {
+	if name == "" || name == "golang" {
+		return utls.ClientHelloID{}, false
+	}
+	id, ok := fingerprintMap[name]
+	return id, ok
+}