Add kubectl-output plugin

[GrigoriyMikhalkin]

kubectl-output is a plugin for kubectl that allows users to set custom output format for specific resources/namespaces.
Custom output format is based on custom-columns.

Example usage:

# create default template
kubectl output set pod --name=test -c=NAME:.metadata.name

# create template for kube-system namespace from file
cat pod.tmpl
# NAME          RSRC
# metadata.name metadata.resourceVersion

kubectl output pod --name=test-file --namespace=kube-system -c=./pod.tmpl

# list all templates for pod
kubectl output template pod -a
# test
# test-file

# get pods with default template
kubectl output get pods -A
# NAME
# pod1

# override default template
kubectl output get pods -A -o test-file
# NAME                 RSRC
# pod1                 123

Plugin repo: https://github.com/GrigoriyMikhalkin/kubectl-output

[ahmetb]

🤖 Beep beep! I’m a robot speaking on behalf of @ahmetb. 🤖

---

Thanks for submitting your kubectl plugin to Krew!
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• Make sure your plugin follows the best practices.
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Thanks for your patience!
/kind new-plugin

[k8s-ci-robot]

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[ahmetb]

Thanks for the submission and apologies for late review. Looks quite nice. Please mention Krew installation method and make sure your --help messages/README are compatible with kubectl output syntax when it's invoked as a plugin.

/lgtm
/approve
Also, please consider setting up Krew release automation which helps you skip manually making updates to your Krew manifests on each new version and send a pull request. It’s a GitHub workflow bot that can run every time you push a new tag. Those PRs are automatically approved.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahmetb, GrigoriyMikhalkin

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