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