feat: utlsFakeCustomExtension in ALPS

- Introducing `utlsFakeCustomExtension` to enable implementation for custom extensions to be exchanged via ALPS.
- currently it doesn't do anything.

Co-Authored-By: Blake Byrnes <115056+blakebyrnes@users.noreply.github.com>

u_common.go

@@ -30,6 +30,7 @@ const (
 	utlsExtensionExtendedMasterSecret uint16 = 23    // https://tools.ietf.org/html/rfc7627
 	utlsExtensionCompressCertificate  uint16 = 27    // https://datatracker.ietf.org/doc/html/rfc8879#section-7.1
 	utlsExtensionApplicationSettings  uint16 = 17513 // not IANA assigned
+	utlsFakeExtensionCustom           uint16 = 1234  // not IANA assigned, for ALPS
 
 	// extensions with 'fake' prefix break connection, if server echoes them back
 	fakeExtensionTokenBinding         uint16 = 24

u_handshake_messages.go

@@ -71,6 +71,7 @@ type utlsClientEncryptedExtensionsMsg struct {
 	raw                    []byte
 	applicationSettings    []byte
 	hasApplicationSettings bool
+	customExtension        []byte
 }
 
 func (m *utlsClientEncryptedExtensionsMsg) marshal() (x []byte) {
@@ -88,6 +89,12 @@ func (m *utlsClientEncryptedExtensionsMsg) marshal() (x []byte) {
 					msg.AddBytes(m.applicationSettings)
 				})
 			}
+			if len(m.customExtension) > 0 {
+				extensions.AddUint16(utlsFakeExtensionCustom)
+				extensions.AddUint16LengthPrefixed(func(msg *cryptobyte.Builder) {
+					msg.AddBytes(m.customExtension)
+				})
+			}
 		})
 	})