[cmd][experimental] Add an example external admission check controller.

[trasc]

What type of PR is this?

/kind documentation
/kind feature

What this PR does / why we need it:

Add an example external admission check controller to provide an easier to follow reference implementation in the documentation.

Which issue(s) this PR fixes:

Fixes #3249

Special notes for your reviewer:

Does this PR introduce a user-facing change?

None

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: trasc
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