Support migrating restricted discovery key to Arti format

It would be neat if arti hsc had a ctor-migrate subcommand (much like arti hss ctor-migrate) for migrating C Tor restricted discovery keys to the Arti keystore.

@deeplow tells me the SecureDrop team is currently using a python script to convert the keys to Arti format.

(I've assigned myself so I don't lose track of this ticket, but it's up for grabs if anyone else wants to take a stab at it)