feat(cli): Add support for Argo CLI plugins

[dinever]

Closes #4158

This will allow us to create CLI plugins in the kubectl style.

For example, by having this at /usr/local/bin/argo-foo

#!/bin/bash

# optional argument handling
if [[ "$1" == "version" ]]
then
    echo "1.0.0"
    exit 0
fi

echo "I am a plugin named argo-foo"

We can do:

This will open opportunities for the community to write Argo plugins for some more specific use cases. For example:

• Proposal: argo cp command to download artifacts #695
• [Proposal] Argo SOLO #794

TODO:

• Add tests
• Write docs for Argo plugin

Thanks!

/cc @alexec

[dinever]

Do we also want a argo plugin list command to list all the available plugins? This is what kubectl has: https://kubernetes.io/docs/tasks/extend-kubectl/kubectl-plugins/#checking-for-plugin-warnings

[codecov]

Codecov Report

Merging #5586 (4bbfd59) into master (4e450e2) will decrease coverage by 0.16%.
The diff coverage is 0.00%.

❗ Current head 4bbfd59 differs from pull request most recent head 883b4b2. Consider uploading reports for the commit 883b4b2 to get more accurate results

@@            Coverage Diff             @@
##           master    #5586      +/-   ##
==========================================
- Coverage   47.01%   46.84%   -0.17%     
==========================================
  Files         240      241       +1     
  Lines       15002    15044      +42     
==========================================
- Hits         7053     7048       -5     
- Misses       7048     7099      +51     
+ Partials      901      897       -4     

Impacted Files	Coverage Δ
cmd/argo/commands/plugin.go	0.00% <0.00%> (ø)
cmd/argo/commands/root.go	0.00% <0.00%> (ø)
server/workflow/workflow_server.go	38.74% <0.00%> (-3.99%)	⬇️
workflow/controller/operator.go	70.96% <0.00%> (+0.27%)	⬆️
cmd/argo/commands/get.go	56.66% <0.00%> (+0.66%)	⬆️
workflow/metrics/server.go	16.66% <0.00%> (+4.16%)	⬆️

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[dinever]

This file is pretty much copied from https://github.com/kubernetes/kubernetes/blob/v1.19.6/pkg/kubectl/cmd/cmd.go#L330-L429

Unfortunately, I couldn't just import it because it was not in the k8s.io/kubectl module until v1.20.x (we are using the v1.19.6 SDK).

One way to use it is to directly import k8s.io/kubernetes which we are not supposed to do.

[dinever]

Closing this for now because as of k8s API v0.19.6 the NewDefaultPluginHandler is not exposed. Will be easier to implement once we upgrade the k8s API to v0.20.x.