Transition of SSH-related implementations

[mih]

Some initial thoughts to be forged into an actionable plan.

We have the new SSHRemoteIO, as a first big step, with the understanding that the goal is to make it obsolete.
Other big pieces are create-sibling-ria and create-sibling.

I am leaning toward providing a completely new create-sibling-ssh from datalad-next, and then patching it in to be used from create-sibling in -core, only leaving a small proxy.

This leaves sshrun. There is nothing really todo wrt to shell, but these pieces together are the users of the "sshconnector". When we replace pieces, we should have a replacement of that code in mind too, and maybe we can inject an intermediate API layer already that allows us to kill the inside a bit simpler later on.