refactor+feat: Custom Client Handshake + Implement ALPS extension (#142)

* refactor: split `CompressCertExtension` changes

- Split most of changes for `CompressCertExtension` made to `crypto/tls` files out and moved them to `u_` files.
- Edited some `crypto/tls` files to achieve better programmability for uTLS.
- Minor styling fix.

* feat: implement ALPS Extension draft

- Made necessary modifications to existing types to support ALPS.
- Ported `ApplicationSettingsExtension` implementation from `ulixee/utls` by @blakebyrnes with some adaptation.

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

* 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>

* fix: magic number in `StatusRequestV2Extension`

- Fixed magic number `17` in `StatusRequestV2Extension` with pre-defined enum `extensionStatusRequestV2`.

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

u_conn.go

@@ -710,3 +710,30 @@ func makeSupportedVersions(minVers, maxVers uint16) []uint16 {
 	}
 	return a
 }
+
+// Extending (*Conn).readHandshake() to support more customized handshake messages.
+func (c *Conn) utlsHandshakeMessageType(msgType byte) (handshakeMessage, error) {
+	switch msgType {
+	case utlsTypeCompressedCertificate:
+		return new(utlsCompressedCertificateMsg), nil
+	case utlsTypeEncryptedExtensions:
+		if c.isClient {
+			return new(encryptedExtensionsMsg), nil
+		} else {
+			return new(utlsClientEncryptedExtensionsMsg), nil
+		}
+	default:
+		return nil, c.in.setErrorLocked(c.sendAlert(alertUnexpectedMessage))
+	}
+}
+
+// Extending (*Conn).connectionStateLocked()
+func (c *Conn) utlsConnectionStateLocked(state *ConnectionState) {
+	state.PeerApplicationSettings = c.utls.peerApplicationSettings
+}
+
+type utlsConnExtraFields struct {
+	hasApplicationSettings   bool
+	peerApplicationSettings  []byte
+	localApplicationSettings []byte
+}