Implement SSL_CONF_cmd Certificate and PrivateKey commands

[cpu]

This branch follows #29, #31 extending the existing SSL_CONF_xxx API to support the "Certificate" and "PrivateKey" sub-commands. See man 3 SSL_CONF_cmd for more information.

We diverge slightly from the upstream here by re-using the existing SSL_CTX_use_certificate_chain_file and SSL_CTX_use_PrivateKey_file logic. In the case of SSL_CTX_use_certificate_chain_file this means pulling out a use_cert_chain_file helper with the existing logic so it can be reused. Taking this approach requires the minimum amount of fuss to support these configuration commands, but means we error earlier in the case of the cert file/private key being bogus. This seems better to me than putting in more extra design work just to delay erroring! Relatedly, we don't have all the pieces needed to support setting a certificate chain by PEM input path for a SSL, just a SSL_CTX, so we skip implementing Certificate after SSL_CONF_CTX_set_ssl - in practice Nginx only uses SSL_CONF_CTX_set_ssl_ctx so we can punt here for the time being.

An integration test using Nginx's ssl_conf_command directive to override the normal ssl_certificate and ssl_certificate_key directives using the Certificate and PrivateKey commands demonstrates the end-to-end implementation working w/ Nginx 1.24+.

Updates #22