Expore DNS tunneling options

During the intensified censorship events, many pluggable transports stopped working with the allow list based approach.

This made it more urgent to explore dns tunneling options which are less impacted by this kind of restrictive network environment.

The following options exists for us to explore:

1. https://endpositive.github.io/slipstream/ (better performance)
2. https://www.bamsoftware.com/software/dnstt/ (written in a convenient language)
3. https://github.com/Mygod/slipstream-rust (better performance, written in a semi convenient language, machine generated code)

DNS Tunneling

When using plaintext dns tunneling, here are the general trade offs:

1. the connection will be relatively slow
2. a passive observer can detect the connection attempt as anomaly