x/crypto/ssh: investigate agent client support on Windows

[drakkan]

The agent client talks to the ssh-agent process over Unix Domain sockets on *NIX.

On Windows, the ssh-agent is implemented as a Windows service and exposed on a named pipe, \.\pipe\openssh-ssh-agent by default.

We currently does not support named pipes in the standard library. I think we can implement this feature once CL 299009 is merged.

cc @golang/security