Add "sample-plugin" staging repo

[soltysh]

Supersedes #67729

Showcases usage of the new kubectl plugins mechanism.
Showcases usage of the new cli-runtime repo from a third-party plugin

See README.md for details on what this plugin actually does.

/assign @smarterclayton @sttts @juanvallejo

/sig cli

Release note:

Adds sample-cli-plugin staging repository

[sttts]

/lgtm

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[soltysh]

/retest
And re-applying the label back after fixing missing meta files.

[smarterclayton]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: smarterclayton, soltysh, sttts

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• hack/OWNERS [smarterclayton,soltysh,sttts]
• staging/OWNERS [smarterclayton]
• vendor/OWNERS [smarterclayton,sttts]

Approvers can indicate their approval by writing /approve in a comment
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[soltysh]

/retest

[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 67938, 66719, 67883). If you want to cherry-pick this change to another branch, please follow the instructions here.