Support remote containerd management

At current version of nerdctl and containerd, we can only manage local containerd instance.

nerdctl cannot manage containerd via remote socket.

How to try remote containerd management:
1. at machine running containerd, execute `./sockfwd -s 0.0.0.0:9090 -d unix:/run/containerd/containerd.sock`.
2. at local machine, execute `./sockfwd -d 123.57.254.88:9090 -s unix:/tmp/containerd.sock`
3. at local machine, execute `./_output/nerdctl --debug -a /tmp/containerd.sock run --network=none --rm -it alpine echo 1` and failed.

https://github.com/robberphex/sockfwd/

I think we need to modify containerd and nerdctl as well.