🌱 Add certwatcher callback

[vincepri]

Fixes #1982

[k8s-ci-robot]

@vincepri: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-controller-runtime-apidiff	aeedfbf	link	false	/test pull-controller-runtime-apidiff

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[vincepri]

cc @ahmetb

[vincepri]

/milestone v0.15.x

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: alvaroaleman, vincepri

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