[Request] Add TUN inbound as env variable for iOS

[fwx]

Xray-core added TUN inbound support for iOS in this PR:
XTLS/Xray-core#5612

Readme states that:

Set the environment variable xray.tun.fd (or XRAY_TUN_FD) to the fd number before starting Xray. This can be done from Swift/Objective-C or by exposing a Go function via gomobile bindings.

I think we could add this into xray_wrapper.go when xray-core gets a new stable release because lots of iOS clients probably use libXray.

Something like this:

# import few more things:
"os"
"strconv"

...

// SetTunFd sets the TUN file descriptor for iOS.
// Call this BEFORE RunXray/RunXrayFromJSON.
func SetTunFd(fd int32) {
	os.Setenv("XRAY_TUN_FD", strconv.Itoa(int(fd)))
}