Support AWS Private CA as CA

[bryanmechetmrs]

Useful as a potential replacement for Acme providers.

[webprofusion-chrisc]

Hi Bryan, thanks for your suggestion. We don't currently have support for any non-ACME issuers but we will keep this open for now.

If they ever add ACME or if something like https://github.com/grindsa/acme2certifier adds support for them then that would be the simplest option (and enable any ACME client to use them).

It's possible we will add support for some non-ACME CAs in the future, but it's not currently planned.