Option to use publicly signed cert vs self generated

[jebucha]

Hello!

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Issue details

We would like the ability to configure step-ca to use a publicly signed cert from say DigiCert vs self-generated.

Why is this needed?

We are deploying via your step-certificates helm chart using an intermediate cert signed by our internal PKI and would like to be able to use a publicly signed cert to avoid needing to distribute the internally generated cert chain to all of our ~4000 ACME clients.