feat: Add support for custom root CA subject - #14

Hi, first of all, thanks for creating minica! Finally a solution that doesn't make my head explode because of cryptic openssl commands.

You're welcome! Thanks for the kind words, and sorry it's taken a while to get back to you.

I added the feature to declare a custom subject for the root CA by providing a JSON file with the necessary fields.

My goal with minica is to keep it radically simple. Right now it generates a random subject CN, as you've probably seen. Customizing the subject beyond that doesn't add compatibility, standards compliance, or features as far as I know, so my inclination is to not incorporate that into the main program. Please let me know if I'm wrong in that evaluation.

However, minica will load any file named minica.pem in the current directory, so you can definitely create your own self-signed root with another program (or, indeed, with a forked and modified copy of minica).

There's still an issue I don't know how to fix: The result looks like OU=,O=Frederic Hemberger,POSTALCODE=,STREET=,L=,ST=,C=DE if not all fields exist in the JSON file. Unfortunately, this gives me a parsing error on macOS because of the empty fields. Is there a way in Go to map only existing fields to avoid this?

You are unconditionally setting these fields to a one-element slice which sometimes contains an empty string:

  PostalCode:         []string{subject.PostalCode},

If you want the PostalCode field (for example) to be empty, you need to set it to nil, or possible an empty slice.

Thanks for your feedback, it helped a lot!

Closing.