✨ Introduce possibility to create clusterctl plugins

[alexander-demicev]

What this PR does / why we need it:

We would like clusterctl to support bootstrapping a management cluster using the CAPI Operator, but at the same time, we'd like to avoid introducing new code to clusterctl for maintenance reasons (dependencies, etc.). One possible option for us is to enable the creation of clusterctl plugins in a similar way to kubectl.

This PR introduces a change that enables the creation of such plugins. The code relies on the kubectl package and borrows some code from it, specifically https://github.com/kubernetes/kubectl/blob/master/pkg/cmd/cmd.go#L115. In the current implementation, it will only be possible to create plugins for the clusterctl commands and not for subcommands.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[fabriziopandini]

I need some time to look at additional transitive dependencies + test; in the meantime, it will be great if you can document plugins in https://cluster-api.sigs.k8s.io/clusterctl/provider-contract.html (might be there is a better place)

[fabriziopandini]

@ykakarap @Jont828 PTAL

[sbueringer]

I need some time to look at additional transitive dependencies + test; in the meantime, it will be great if you can document plugins in https://cluster-api.sigs.k8s.io/clusterctl/provider-contract.html (might be there is a better place)

sigs.k8s.io/kustomize/kustomize/v5 v5.0.1 seems to be a big one

[Fedosin]

Tested this PR locally and it seems working right.
@alexander-demicev let me know if you need any help with documentation or tests for this patch.

[fabriziopandini]

Dependency seems ok (they are all already k8s deps)

The only tricky one is sigs.k8s.io/kustomize/kustomize/v5 v5.0.1, which is big one, so I will ask other maintainers for an opinion on it (I'm +1)

Also, the PR is lacking the necessary doc + test, so we should consider if to get this merged before code-freeze if @alexander-demicev can follow up in the next two weeks at least for docs (I'm +1)

cc @CecileRobertMichon @sbueringer @killianmuldoon @enxebre @vincepri

[vincepri]

I'm +1 as well

[CecileRobertMichon]

no objections on dependencies from my end

cc @dtzar @mboersma

[alexander-demicev]

I added some documentation, most of what is described in kubectl docs also applies here.

[Fedosin]

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 3088c09ace205b3ce91afaee4810f7f1fd06d9db

[vincepri]

/approve

[k8s-ci-robot]

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[vincepri]

Thank you for doing this!

[sbueringer]

/lgtm