Finalmask: Refactor header conns to avoid multiple-copy; Add randRange to "header-custom" (TCP & UDP) (#5812)

https://github.com/XTLS/Xray-core/pull/5657#issuecomment-4016760602
https://github.com/XTLS/Xray-core/pull/5657#issuecomment-4052921628

common/crypto/crypto.go

@@ -4,8 +4,11 @@ package crypto // import "github.com/xtls/xray-core/common/crypto"
 import (
 	"crypto/rand"
 	"math/big"
+
+	"github.com/xtls/xray-core/common"
 )
 
+// [,)
 func RandBetween(from int64, to int64) int64 {
 	if from == to {
 		return from
@@ -16,3 +19,20 @@ func RandBetween(from int64, to int64) int64 {
 	bigInt, _ := rand.Int(rand.Reader, big.NewInt(to-from))
 	return from + bigInt.Int64()
 }
+
+// [,]
+func RandBytesBetween(b []byte, from, to byte) {
+	common.Must2(rand.Read(b))
+
+	if from > to {
+		from, to = to, from
+	}
+
+	if to-from == 255 {
+		return
+	}
+
+	for i := range b {
+		b[i] = from + b[i]%(to-from+1)
+	}
+}