Implement SSL_CONF_*

[cpu]

In #20 we stubbed out the SSL_CONF_* API surface used by Nginx. There's a lot of functionality behind these functions, but it looks like it would be important/reasonable to implement support for the options that have obvious rustls config counterparts.

[cpu]

cpu self-assigned this now

I've started to work on this.

[cpu]

We've implemented support for MinProtocol, MaxProtocol, VerifyMode, Certificate, PrivateKey, VerifyCAPath and VerifyCADir.

There are other commands we could implement once additional infrastructure is in place (see this comment) but for now I think it makes sense to call this "Done Enough ™️" and focus on other areas until there's a reason to re-prioritize.