[Feature Request] Return Namespaces from Cache to STDOUT.

[sherifabdlnaby]

Currently, the command switch ns cannot take a namespace as an input. It always opens the interactive list.
It will be good to implement `switch list namespaces that will print the list of "cached" namespaces.

This can be used to generate auto-completion for any commands including switch itself, but can also be used with other commands such as k9s -n <auto-complete> where auto-complete comes from switch.