feat: add DSCP rule for Tproxy UDP packets (#996)

* feat: add `DSCP` rule for Tproxy UDP packets

* fix: fix compatibility issue with non_linux platform

* chore: remove redundant lines for DSCP

constant/metadata.go

@@ -147,6 +147,7 @@ type Metadata struct {
 	SpecialProxy string     `json:"specialProxy"`
 	SpecialRules string     `json:"specialRules"`
 	RemoteDst    string     `json:"remoteDestination"`
+	DSCP         uint8      `json:"dscp"`
 
 	RawSrcAddr net.Addr `json:"-"`
 	RawDstAddr net.Addr `json:"-"`

constant/rule.go

@@ -14,6 +14,7 @@ const (
 	SrcPort
 	DstPort
 	InPort
+	DSCP
 	InUser
 	InName
 	InType
@@ -73,6 +74,8 @@ func (rt RuleType) String() string {
 		return "RuleSet"
 	case Network:
 		return "Network"
+	case DSCP:
+		return "DSCP"
 	case Uid:
 		return "Uid"
 	case SubRules: