Add shell completion for new --subresource flag

[marckhouzam]

What type of PR is this?

/kind cleanup
/sig cli

What this PR does / why we need it:

Adds shell completion for the new --subresource flag.

#99556 added the --subresource flag which accepts a specific set of values, mostly status and scale.
The help message of the flag is clear on what values can be accepted.
I feel that having the shell completion provide those choices can help the user even further.
Currently, if users try to complete the values of the --subresource flag, they will get files being suggested.

Currently:

$ kubectl get --subresource s<TAB>
SECURITY_CONTACTS  SUPPORT.md         staging/

With PR:

$ /tmp/kubectl get --subresource s<TAB>
scale   status

Does this PR introduce a user-facing change?

Shell completion is now provided for the "--subresource" flag.

/cc @nikhita

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/lifecycle stale

[marckhouzam]

Not very important but still applicable

[k8s-triage-robot]

The Kubernetes project currently lacks enough active contributors to adequately respond to all issues and PRs.

This bot triages issues and PRs according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
• After 30d of inactivity since lifecycle/rotten was applied, the issue is closed

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/lifecycle rotten

[marckhouzam]

Not very important but since it's only a 3-line change maybe @brianpursley has time to take a look?

[brianpursley]

Sure I'll take a look...

/remove-lifecycle rotten
/assign

[brianpursley]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: brianpursley, marckhouzam

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