Proxy: Add TUN inbound for Windows & Linux, including Android (#5464)

* Proxy: Implement tun raw network interface inbound support for Linux

* Proxy: Tun. Include "android" as build condition for build of tun_default implementation

* Proxy: Tun. Add .Close() cleanup calls to Handler.Init() where needed

* Proxy: Add Tun for Android

* Proxy: Tun. Implement Windows support

---------

Co-authored-by: yuhan6665 <1588741+yuhan6665@users.noreply.github.com>

proxy/tun/tun_android.go

@@ -0,0 +1,58 @@
+//go:build android
+
+package tun
+
+import (
+	"context"
+	"strconv"
+
+	"github.com/xtls/xray-core/common/errors"
+	"github.com/xtls/xray-core/common/platform"
+	"golang.org/x/sys/unix"
+	"gvisor.dev/gvisor/pkg/tcpip/link/fdbased"
+	"gvisor.dev/gvisor/pkg/tcpip/stack"
+)
+
+type AndroidTun struct {
+	tunFd   int
+	options TunOptions
+}
+
+// DefaultTun implements Tun
+var _ Tun = (*AndroidTun)(nil)
+
+// DefaultTun implements GVisorTun
+var _ GVisorTun = (*AndroidTun)(nil)
+
+// NewTun builds new tun interface handler
+func NewTun(options TunOptions) (Tun, error) {
+	fd, err := strconv.Atoi(platform.NewEnvFlag(platform.TunFdKey).GetValue(func() string { return "0" }))
+	errors.LogInfo(context.Background(), "read Android Tun Fd ", fd, err)
+
+	err = unix.SetNonblock(fd, true)
+	if err != nil {
+		_ = unix.Close(fd)
+		return nil, err
+	}
+
+	return &AndroidTun{
+		tunFd:   fd,
+		options: options,
+	}, nil
+}
+
+func (t *AndroidTun) Start() error {
+	return nil
+}
+
+func (t *AndroidTun) Close() error {
+	return nil
+}
+
+func (t *AndroidTun) newEndpoint() (stack.LinkEndpoint, error) {
+	return fdbased.New(&fdbased.Options{
+		FDs:               []int{t.tunFd},
+		MTU:               t.options.MTU,
+		RXChecksumOffload: true,
+	})
+}