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>

key_agreement.go

@@ -319,7 +319,7 @@ func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHell
 		}
 
 		if !isSupportedSignatureAlgorithm(signatureAlgorithm, clientHello.supportedSignatureAlgorithms) {
-			return errors.New("tls: certificate used with invalid signature algorithm")
+			return fmt.Errorf("tls: certificate used with invalid signature algorithm -- ClientHello not advertising %04x", uint16(signatureAlgorithm))
 		}
 		sigType, sigHash, err = typeAndHashFromSignatureScheme(signatureAlgorithm)
 		if err != nil {