Define a link specifier containing an ntor onion key
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