Draft: Vendorize Golang source from ingress-nginx with git subtree

What is your Merge about?

Refactor to vendorize the kube-webhook-certgen Golang source from ingress-nginx with git subtree. Ingress-nginx has not been consistent in their updates of the container on registry.k8s.io which leaves long periods of this container having Golang related vulnerabilities.

This approach allows pulling in changes to ingress-nginx quickly and updating the version of Golang from the Dockerfile thus simplifying future maintenance.

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