perf: added constant for VpnVore

pb/constant/vpncore.go

@@ -0,0 +1,8 @@
+package constant
+
+type VpnCore string
+
+const (
+	VpnCoreXray    VpnCore = "Xray"
+	VpnCoreSingBox VpnCore = "SingBox"
+)