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>

tls_test.go

@@ -12,7 +12,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"github.com/refraction-networking/utls/testenv"
 	"io"
 	"math"
 	"net"
@@ -22,6 +21,8 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/refraction-networking/utls/testenv"
 )
 
 var rsaCertPEM = `-----BEGIN CERTIFICATE-----
@@ -827,6 +828,8 @@ func TestCloneNonFuncFields(t *testing.T) {
 			f.Set(reflect.ValueOf(RenegotiateOnceAsClient))
 		case "mutex", "autoSessionTicketKeys", "sessionTicketKeys":
 			continue // these are unexported fields that are handled separately
+		case "ApplicationSettings":
+			f.Set(reflect.ValueOf(map[string][]byte{"a": {1}}))
 		default:
 			t.Errorf("all fields must be accounted for, but saw unknown field %q", fn)
 		}