✨ feat(CLI): Add completion for the plugins flag

[vitorfloriano]

This PR adds dynamic flag completion for the --plugins flag.

The --plugins flag now shows completion for all the available plugins (external ones too, including out-of-tree! 🎉), while hiding deprecated ones.

$ kubebuilder init --plugins 
autoupdate.kubebuilder.io/v1-alpha       (Proposes Kubebuilder scaffold updates via GitHub Actions)
base.go.kubebuilder.io/v4                (Default scaffold (go/v4 + kustomize/v2))
deploy-image.go.kubebuilder.io/v1-alpha  (Scaffolds a CRD+controller to deploy an image-based Operand)
foo.acme.io/v2                           (External or custom plugin)
go.kubebuilder.io/v4                     (Default scaffold (go/v4 + kustomize/v2))
grafana.kubebuilder.io/v1-alpha          (Generates Grafana Dashboards for metrics)
helm.kubebuilder.io/v2-alpha             (Generates a Helm chart for project distribution)
kustomize.common.kubebuilder.io/v2       (Scaffolds base Kustomize configuration)

This will improve plugin discoverability and usage, especially for the autoupdate plugin.

Relates to #5446 and #5291

Note

Completion does not work for comma-chained values, so users will either use another instance of --plugins to get completion or type everything (or copy-paste) the keys after a comma:

Works (multiple instances of the flag):

$ kubebuilder init --plugins=foo.acme.io/v2 --plugins autoupdate.kubebuilder.io/v1-alpha --plugins base.go.kubebuilder.io/v4 --plugins helm.kubebuilder.io/v2-alpha --plugins 
deploy-image.go.kubebuilder.io/v1-alpha  (Scaffolds a CRD+controller to deploy an image-based Operand)
go.kubebuilder.io/v4                     (Default scaffold (go/v4 + kustomize/v2))
grafana.kubebuilder.io/v1-alpha          (Generates Grafana Dashboards for metrics)
kustomize.common.kubebuilder.io/v2       (Scaffolds base Kustomize configuration)

Does not work (trailing comma):

$ kubebuilder init --plugins foo.acme.io/v2,<TAB><TAB><TAB furiously>
(nothing happens)
$ kubebuilder init --plugins=foo.acme.io/v2,<TAB><TAB><TAB furiously>
(nothing happens)

This is a known limitation in Cobra. There are workarounds but they look a bit of a kludge.

[k8s-ci-robot]

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[camilamacedo86]

The completion should work as it works the plugins
So, it should work well with --plugins=pluginA,pluginB,pluginB

Could we ensure that?
Also, do we really need this note here?
If yes, could we add the default style note instead [!TIP]?