Extend arti hss ctor-migrate to support ad-hoc migrations

The arti hss ctor-migrate has two use cases:

migrating C Tor keys from a configured C Tor keystores into the Arti native keystore (implemented in !3102 (merged))
"ad-hoc migrations" from a given C Tor keystore, given as a path on disk (less straightforward, will require some changes to edit the storage.keystore config/config builder to include the keystore path specified via --from. Alternatively, we could just build an ad-hoc CTorServiceKeystore in the CLI, but ideally we'd have a higher level InertTorClient API for this. We'll also need to require the --nickname flag in this case, because otherwise there's no way for the CLI to infer which service the keys are being imported for)

This ticket is about implementing ad-hoc migrations.