Implement authenticated interactions with the Registry

[f-f]

Spago should be able to deal with package transfers, and unpublishing package versions.
This requires us to implement the authentication mechanism described in the registry spec

We'd add new spago registry transfer and spago unpublish commands to execute these operations, and we'll need to implement the SSH auth bits that power these interactions. It will also require implementing the owner key in the config (see purescript/registry-dev#594).

We'll need to use the ssh2 package to implement all this - we already depend on it in fact, and it's included in the bundle already, so that part is done at least.