Teach HTTPT to register itself to rdsys and forward traffic to a Tor bridge

This is a child issue of #1 (moved).

If we run an HTTPT server independently from a Tor bridge, we need to teach it how it can register itself to rdsys, so we can hand out HTTPT proxies to users. Over at tpo/anti-censorship/rdsys#4 (closed), we are brainstorming an API that allows systems like HTTPT to register themselves.

In addition to the registration mechanism, we need to teach HTTPT to forward traffic to a bridge. For now, we may want to set up a dedicated and centralised bridge, just like the one we are using in Snowflake.