Make the CA details and filename configurable

Problem Description

I'm wrapping mitmproxy and I want to avoid confusion by not calling my root certs mitmproxy-ca.pem etc. And also by not having my root cert be issued by mitmproxy. So that if you're also using mitmproxy it's clear which is which.

Proposal

CONF_BASENAME looks like it's almost meant to be an option here:

https://github.com/mitmproxy/mitmproxy/blob/8f23a26e355c472725206839125be9d51f548b10/mitmproxy/options.py#L7

and the organization and cn arguments are already there, but unused in our codebase:

https://github.com/mitmproxy/mitmproxy/blob/8f23a26e355c472725206839125be9d51f548b10/mitmproxy/certs.py#L393

I think having these three things be configurable would already be amazing.

Would we want something like ca_basename, ca_organization and ca_cn in core?

Alternatives

I'm aware of https://docs.mitmproxy.org/stable/concepts-certificates/#using-a-custom-certificate-authority but I'd rather have mitmproxy manage the cert for me. It would also still look for the same filename.