If we want to support intro and rend points that aren't in the consensus, we need to define and parse a link specifier with an ntor onion key in it. In order to extend via ntor to an introduction point, either: * the introduction point must be in the consensus, and have an ntor onion key in its descriptor, or * the HS descriptor must contain the ntor onion key for the introduction point In order to extend via ntor to a rendezvous point, either: * the rendezvous point must be in the consensus, and have an ntor onion key in its descriptor, or * the INTRODUCE2 cell must contain the ntor onion key for the rendezvous point