Tunnel Mode Support

[brianwoo]

Please read contributing guidelines. Thanks.

Is your feature request related to a problem? Please describe.
I have an application that runs on a server but it's listening on localhost only (for security reasons). I can access this application with ss-tunnel on PC, but I have yet figured a way to make this work on Android.

Describe the solution you'd like
Currently the Android app supports socks5 mode, transparent proxy mode and I would like to have tunnel mode support as well.

Describe alternatives you've considered
I don't have any alternative at this point.

Additional context
N/A

[kataliyst]

Also wanting this feature. It is useful in Windows for our applications, so would appreciate seeing it on Android.