Hide the default completion command of plugins

[marckhouzam]

What this PR does / why we need it

Plugins automatically get a default completion command from Cobra. For example:

tanzu cluster completion
tanzu package completion

This commit hides this default command for any plugin through the plugin runtime library.

Notice that users enable shell completion using tanzu completion, and that this allows completion to work for plugins as well. Therefore plugins do not need to also have their own completion command. Having such a command for a plugin (e.g., tanzu cluster completion) is just confusing for users.

Which issue(s) this PR fixes

Fixes #3160

Describe testing done for PR

I built the CLI and the plugins and then ran

$ tanzu cluster -h
$ tanzu management-cluster -h
$ tanzu package -h

and confirmed there was no completion command listed.

I then ran

$ tanzu cluster completion
$ tanzu management-cluster completion
$ tanzu package completion

and confirmed the command still existed but is simply hidden.
The reasons for keeping it hidden instead of removing it is explained in the section below.

Release note

The completion command of any plugin is removed since it is not needed.

Additional information

We don't completely disable a plugin's default completion command for two reasons:

1. backwards-compatibility, as the command used to be available for some plugins
2. to allow to use shell completion when using the plugin as a native program (mostly for testing)

Note that a plugin can completely disable this command itself using:

plugin.Cmd.CompletionOptions.DisableDefaultCmd = true

Special notes for your reviewer

This change impacts all plugins that will be compiled with this new version of the runtime library.

[marckhouzam]

Although the code is ready I've left this PR as a draft because is like to discuss if plugin owners need to be consulted

[codecov]

Codecov Report

Merging #3888 (43c8e8c) into main (2f9044a) will decrease coverage by 0.89%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #3888      +/-   ##
==========================================
- Coverage   46.93%   46.03%   -0.90%     
==========================================
  Files         402      427      +25     
  Lines       40203    41765    +1562     
==========================================
+ Hits        18869    19227     +358     
- Misses      19582    20768    +1186     
- Partials     1752     1770      +18     

Impacted Files	Coverage Δ
cli/runtime/plugin/root.go	100.00% <100.00%> (ø)
...ons/controllers/packageinstallstatus_controller.go	77.99% <0.00%> (-1.16%)	⬇️
cmd/cli/plugin/cluster/main.go	0.00% <0.00%> (ø)
cmd/cli/plugin/cluster/kubeconfig_get.go	8.82% <0.00%> (ø)
...cluster/delete_machinehealthcheck_control_plane.go	16.66% <0.00%> (ø)
cmd/cli/plugin/cluster/delete.go	12.50% <0.00%> (ø)
cmd/cli/plugin/cluster/set_machinehealthcheck.go	23.33% <0.00%> (ø)
cmd/cli/plugin/cluster/set_node_pool.go	14.63% <0.00%> (ø)
cmd/cli/plugin/cluster/list.go	11.36% <0.00%> (ø)
cmd/cli/plugin/cluster/get_machinehealthcheck.go	11.42% <0.00%> (ø)
... and 17 more

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[marckhouzam]

This is ready for review

[anujc25]

This looks good.