support access to ssh remotes

[yarikoptic]

• based on information from annex info and git remotes we can discover which known to annex ssh remotes are already enabled (well, added as git remotes)
  • so for any annexed key we can pretty much mint the "ssh URL" on remote location (well -- paying attention on either it is bare or regular repo -- would matter which annex keystore lead directories naming is used... we dealt with that recently, forgot where... uff)
• we could use datalad/support/sshconnector.py:SSHManager().get_connection(..) to establish the connection to remote host (with mux socket)
  • probably issue warning if that SSHManager is not MultiplexSSHConnection and not bother then supporting ssh "urls"
• there is sftp support in fsspec: https://filesystem-spec.readthedocs.io/en/latest/_modules/fsspec/implementations/sftp.html .
  • Uses paramiko . that one doesn't yet support mux'ing BUT Implement support for ControlMaster / ControlPath / etc paramiko/paramiko#852 (comment) suggests that for sftp it could be easily done
  • so we should be able to instantiate sftp connection using mux established by datalad's SSHManager

WDYT @jwodder ?