Simplify --net ns: syntax?

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

Currently, we require absolute path to the ns.

eg:
```
sudo nerdctl run -d --net ns:/run/netns/lepton-test busybox sleep Inf
```

and (of course?)
```
sudo nerdctl run -d --net ns:lepton-test busybox sleep Inf
```
... will fail.

Is there a particular reason not to allow network namespaces names instead of full path?
Put otherwise: are there cases where passing along a full path that is not under `/run/netns` would be required?

### Describe the solution you'd like

na

### Additional context

_No response_