Implement conjure as a pluggable transport

Get the refraction networking client's code into a pluggable transport for Tor.

• 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

Any new issue to convert it into a PT will live in the new repository:

• Decide how we're going to do signaling -- maybe by reusing our Fastly domain fronting thing, if we don't like the signaling component Conjure uses now.
• Integrate into Tor Browser (including reproducible build)
• Verify that obfs4+conjure actually works
• Deploy bridge somewhere to receive traffic from already setup infrastructure
• Make sure we can still get our extorport-style metrics: if the conjure infra de-obfs4's the traffic, do we lose the features of the extorport, like passing along the country of the original user? If yes fix it somehow.