Feature: export key to PKCS8 (universal) format

[awoimbee]

Some libraries sadly don't support ECC keys/certs, like rustls: rustls/rustls#767.

I think it would be good for this program to support the universal PKCS8 format.

BTW cool project, thanks !

[vsliouniaev]

This repository is no longer maintained. Please submit your changes here instead: https://github.com/kubernetes/ingress-nginx/tree/main/images/kube-webhook-certgen