Write proposals to allow an external program that discovers bridge addresses to tell Tor about them and start implementing the proposals

This ticket is part of sponsor F deliverable 7. Once we have a recommended approach for an external program that discovers bridge addresses to tell Tor about them, we need to write proposals and start implementing them. The "and start implementation as needed" part of the deliverable text is vague enough to focus on the design discussion and proposals. If there's time left to implement something, great, but if not, that's fine, too. We can still implement proposals between March and November 2012.