Support --sig-proxy in run and attach

### What is the problem you're trying to solve

```--sig-proxy``` is supported in ```docker run``` and ```docker attach``` but not in ```nerdctl run``` and ```nerdctl attach```. This can also help to unblock #392

### Describe the solution you'd like

Support ```--sig-proxy``` in ```nerdctl run``` and ```nerdctl attach```.

### Additional context

- ```--sig-proxy``` is listed as unimplemented under [```nerdctl run```](https://github.com/containerd/nerdctl/blob/main/docs/command-reference.md#whale-blue_square-nerdctl-run) and [```nerdctl attach```](https://github.com/containerd/nerdctl/blob/main/docs/command-reference.md#whale-blue_square-nerdctl-run)

- [Docker attach docs](https://docs.docker.com/reference/cli/docker/container/attach/)
- [Docker run docs](https://docs.docker.com/reference/cli/docker/container/run/)