TUN inbound: Add gateway, dns, autoSystemRoutingTable, autoOutboundsInterface for Windows (#5887)

And refactor `mtu` to support setting IPv4/v6 separately

Example: https://github.com/XTLS/Xray-core/pull/5887#issue-4198837696

proxy/tun/config.go

@@ -1 +1,78 @@
 package tun
+
+import (
+	"context"
+	"net"
+	"sync"
+
+	"github.com/xtls/xray-core/common/errors"
+)
+
+type InterfaceUpdater struct {
+	sync.Mutex
+
+	tunIndex  int
+	fixedName string
+	iface     *net.Interface
+}
+
+var updater *InterfaceUpdater
+
+func (updater *InterfaceUpdater) Get() *net.Interface {
+	updater.Lock()
+	defer updater.Unlock()
+
+	return updater.iface
+}
+
+func (updater *InterfaceUpdater) Update() {
+	updater.Lock()
+	defer updater.Unlock()
+
+	if updater.iface != nil {
+		iface, err := net.InterfaceByIndex(updater.iface.Index)
+		if err == nil && iface.Name == updater.iface.Name {
+			return
+		}
+	}
+
+	updater.iface = nil
+
+	interfaces, err := net.Interfaces()
+	if err != nil {
+		errors.LogInfoInner(context.Background(), err, "[tun] failed to update interface")
+		return
+	}
+
+	var got *net.Interface
+	for _, iface := range interfaces {
+		if iface.Index == updater.tunIndex {
+			continue
+		}
+		if updater.fixedName != "" {
+			if iface.Name == updater.fixedName {
+				got = &iface
+				break
+			}
+		} else {
+			addrs, err := iface.Addrs()
+			if err != nil {
+				continue
+			}
+			if (iface.Flags&net.FlagUp != 0) &&
+				(iface.Flags&net.FlagLoopback == 0) &&
+				len(addrs) > 0 {
+				got = &iface
+				break
+			}
+		}
+	}
+
+	if got == nil {
+		errors.LogInfo(context.Background(), "[tun] failed to update interface > got == nil")
+		return
+	}
+
+	updater.iface = got
+	errors.LogInfo(context.Background(), "[tun] update interface ", got.Name, " ", got.Index)
+}