Implement conjure as a pluggable transport

Get the refraction networking client's code into a pluggable transport for Tor. This issue tracks an MVP version of Conjure that is suitable to ship in alpha versions of Tor Browser.

• get familiar with the Go code https://github.com/refraction-networking/gotapdance
• create conjure repo in https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports for this new PT - https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/conjure
• implement a staging environment for testing (#3 (closed))
• implement a working PT (#1 (closed), #2 (closed))
• connect PT to production Conjure station (#6 (closed))
• deploy a Conjure bridge
• secure the connection between the station and the bridge (#15 (closed))
• reproducible builds in tor-browser-build (#14 (closed))
• get IPv6 working (#13)
• save phantom registrations for later use (#7)
• protect the registration with either decoy routing or domain fronting