Build API that lets resources register themselves

Rdsys supports both a push and pull-based model to ingest resources. So far, we only read Tor's cached-extrainfo file (which comes from the bridge authority). Let's build an API that allows systems like HTTPS Proxy to register themselves. There's a diagram over at #3 (closed), which shows the big picture:

This issue concerns the arrow that's labeled with "Publish bridge line."