[k9s] allow `k9s -n` to list all namespaces

[Alan Syue]

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What CLI tool does this relate to?

k9s

Which statement makes the most sense?

This completion spec isn't complete

Issue Details

When I use k9s -n commands, I found we should support it to list all namespaces to make users can choose the namespace they want to use easily.

Maybe we can use kubectl command to find all namespaces.
reference: https://kubernetes.io/docs/concepts/overview/working-with-objects/namespaces/#viewing-namespaces

kubectl get namespace