add completion support for namespace

[silenceshell]

Add namespace completion supports for istioctl.

$ istioctl -i <tab><tab>
istio-system          kube-public          istio-demo          metallb-system
$ istioctl -n <tab><tab>
istio-system          kube-public          istio-demo          metallb-system

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This PR needs cobra 1.2.0 for the PR RegisterFlagCompletionFunc as a global var not working in multi-threaded programs, to fix concurrent map writes errors of tests in parallel of istio(here is an example, this is introduced by #28434).
And as cobra 1.2.0 add a new feature Automatically adds completion command for shell completions, this PR will also add a new command istioctl completion. I will replace istioctl collateral --bash/--zsh to istioctl completion bash/zsh in another PR later.

Pull Request Attributes

Please check any characteristics that apply to this pull request.

[ ] Does not have any user-facing changes. This may include CLI changes, API changes, behavior changes, performance improvements, etc.

[istio-policy-bot]

🤔 🐛 You appear to be fixing a bug in Go code, yet your PR doesn't include updates to any test files. Did you forget to add a test?

Courtesy of your friendly test nag.

[shamsher31]

LGTM, But my only concern is the cobra version upgrade. Not sure if it will break something which is not in the test.

I will let others provide their feedback.

[howardjohn]

I couldn't get this working locally using zsh, but maybe just my environment.. LGTM

[ericvn]

I did get this working locally (Mac using zsh) by doing a make istioctl followed by a source <(./out/darwin_amd64/istioctl completion zsh) and then did ./out/darwin_amd64/istioctl -n<tab><tab>