chore: add cnpg plugin for kubectl

[sxd]

Add the cnpg plugin for kubectl to be used with CloudNativePG operator

Description

This PR aims to add the cnpg pluging for kubectl to be used with CloudNativePG operator, in a future
PR we hope to provide also a way to install this operator using arkade

Motivation and Context

The cnpg plugin can make use of Arkade to distribute the plugin

Issue #852

How Has This Been Tested?

Local laptop using amd64 and RaspberryPi B4 both to install the plugin both with Linux
Local laptop using amd64 with macOS

Are you a GitHub Sponsor yet (Yes/No?)

• Yes
• No

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Documentation

• I have updated the list of tools in README.md if (required) with ./arkade get -o markdown
• I have updated the list of apps in README.md if (required) with ./arkade install --help

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s

• I have tested this on arm, or have added code to prevent deployment

[alexellis]

@Jasstkn could you help out here with explaining how to do the checker script that shows the arch for each binary? The one we have in the contributing guide.

[Jasstkn]

@sxd Hi! thanks for the contribution. Please, check our contribution guide to see how the code should be properly tested.
One of the mentioned ways is to use our test script. You can run it by executing the following:

./hack/test-tool.sh <tool_name>

The output of this should be added to the PR's description. It will speed up the review process 🚀

[sxd]

@alexellis @Jasstkn working on it! thank you!!! and sorry for wasting your time I thought that I followed all the contribution guide but clearly I didn't, my bad! fixing!

[sxd]

@Jasstkn here is the output:

./hack/test-tool.sh kubectl-cnpg
+ ./arkade get kubectl-cnpg --arch arm64 --os darwin --quiet
+ file /home/zeus/.arkade/bin/kubectl-cnpg
/home/zeus/.arkade/bin/kubectl-cnpg: Mach-O 64-bit arm64 executable, flags:<|DYLDLINK|PIE>
+ rm /home/zeus/.arkade/bin/kubectl-cnpg
+ echo

+ ./arkade get kubectl-cnpg --arch x86_64 --os darwin --quiet
+ file /home/zeus/.arkade/bin/kubectl-cnpg
/home/zeus/.arkade/bin/kubectl-cnpg: Mach-O 64-bit x86_64 executable
+ rm /home/zeus/.arkade/bin/kubectl-cnpg
+ echo

+ ./arkade get kubectl-cnpg --arch x86_64 --os linux --quiet
+ file /home/zeus/.arkade/bin/kubectl-cnpg
/home/zeus/.arkade/bin/kubectl-cnpg: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, Go BuildID=c8EnbaDLyYSH5tuhAN-I/0Xl4aey1cq1x8eY1BR2z/AT7ShFvoaMOD48e969WV/tIKr3LQQwdHlqO9VFZ0h, stripped
+ rm /home/zeus/.arkade/bin/kubectl-cnpg
+ echo

+ ./arkade get kubectl-cnpg --arch arm64 --os linux --quiet
+ file /home/zeus/.arkade/bin/kubectl-cnpg
/home/zeus/.arkade/bin/kubectl-cnpg: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, Go BuildID=leVwSvJ3d-6YzsMXcO1B/xnofTK4v6MCNdYr9wwHt/tPTd9mv1MB_orY3ZalS3/yknIaaHoN4vYgohfdfV5, stripped
+ rm /home/zeus/.arkade/bin/kubectl-cnpg
+ echo

+ ./arkade get kubectl-cnpg --arch x86_64 --os ming --quiet
+ file /home/zeus/.arkade/bin/kubectl-cnpg
/home/zeus/.arkade/bin/kubectl-cnpg: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, Go BuildID=leVwSvJ3d-6YzsMXcO1B/xnofTK4v6MCNdYr9wwHt/tPTd9mv1MB_orY3ZalS3/yknIaaHoN4vYgohfdfV5, stripped
+ rm /home/zeus/.arkade/bin/kubectl-cnpg
+ echo

I ran make e2e and I got an error:

    --- FAIL: Test_CheckTools/Download_of_kubeseal (0.47s)

I think it's not related but it's looking for a file that doesn't exists here:

=== NAME  Test_CheckTools/Download_of_kubeseal
    url_checker_test.go:51: Error with HTTP HEAD for kubeseal, https://github.com/bitnami-labs/sealed-secrets/releases/download/helm-v2.7.6/kubeseal-helm-v2.7.6-linux-amd64.tar.gz: status code: 404, body: 

[alexellis]

The e2e tests were failing but are now fixed in the master branch.

@Jasstkn can you give a review when you have time?

[sxd]

I just rebased the PR on master branch so now it shouldn't fail

[Jasstkn]

LGTM

[alexellis]

We just missed the update to the README which is part of the PR template.

You checked it but I don't think you added the file?

I have updated the list of tools in README.md if (required) with ./arkade get -o markdown

[alexellis]

I'll sort out the README for you.

Thanks for contributing to arkade and for being a sponsor too.