XHTTP transport: New options for bypassing CDN's detection (#5414)

Usage: https://github.com/XTLS/Xray-core/pull/5414#issuecomment-3770071786

Closes https://github.com/XTLS/Xray-core/issues/4346

---------

Co-authored-by: 风扇滑翔翼 <Fangliding.fshxy@outlook.com>

common/utils/padding.go

@@ -0,0 +1,24 @@
+package utils
+
+import (
+	"math/rand/v2"
+)
+
+var (
+	// 8 ÷ (397/62)
+	h2packCorrectionFactor = 1.2493702770780857
+	base62TotalCharsNum    = 62
+	base62Chars            = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
+)
+
+// H2Base62Pad generates a base62 padding string for HTTP/2 header
+// The total len will be slightly longer than the input to match the length after h2(h3 also) header huffman encoding
+func H2Base62Pad[T int32 | int64 | int](expectedLen T) string {
+	actualLenFloat := float64(expectedLen) * h2packCorrectionFactor
+	actualLen := int(actualLenFloat)
+	result := make([]byte, actualLen)
+	for i := range actualLen {
+		result[i] = base62Chars[rand.N(base62TotalCharsNum)]
+	}
+	return string(result)
+}