Some use issue about `WithArgs` in namespace_opts.go

[Zheaoli]

Hello all,

I got some special issue about WithArgs in namespace_opts.go. For right now, the code below

func WithArgs(k, v string) NamespaceOpts {
	return func(c *Namespace) error {
		c.args[k] = v
		return nil
	}
}

Just allow people to use string as value when calling this function. But in the specification, the value may be a slice type. FYI https://www.cni.dev/docs/conventions/#args-in-network-config

I'm not sure if there is some mistake?

[Zheaoli]

#14