Update TLS configuration API

[mempirate]

It would be nice to offer this stuff as some easy to set up options on the server and the client in a follow-up, like tcp_tls::config::Server::default().enable_mtls() etc etc

Originally posted by @mempirate in #94 (comment)

[thedevbirb]

Note that the enable_mtls setting would need to applied on the Acceptor struct created from openssl library. As such we can provide a wrapper with an easier API. Such wrapper would probably be needed anyway when we add also support for rustls.