Allow to specify certificates for client authentication

[silvioprog]

Hi.

Do you have plans to allow to specify certificates (.pem, .pfx, .p12 etc.) for client authentication?

I did a small change in file streams.rs (line 51) just to test a connection which requires a PKCS12 certificate:

...
    #[cfg(feature = "tls")]
    fn connect_tls(
        host: &str,
        port: u16,
        connect_timeout: Duration,
        read_timeout: Duration,
    ) -> Result<TlsStream<TcpStream>> {
        use native_tls::Identity;
        let mut builder = TlsConnector::builder();
        let buf = std::fs::read("/home/user/certificate.pfx")?;
        let pkcs12 = Identity::from_pkcs12(&buf, "123456").unwrap();
        builder.identity(pkcs12);
        builder.danger_accept_invalid_certs(true);
        let connector = builder.build()?;
        let stream = BaseStream::connect_tcp(host, port, connect_timeout, read_timeout)?;
        let tls_stream = match connector.connect(host, stream) {
            Ok(stream) => stream,
            Err(HandshakeError::Failure(err)) => return Err(err.into()),
            Err(HandshakeError::WouldBlock(_)) => panic!("socket configured in non-blocking mode"),
        };
        Ok(tls_stream)
    }
...

and it worked fine.

(The danger_accept_invalid_certs(true) is related to #38).

[adamreichold]

This is now possible when using the rustls backend as we expose the full ClientConfig. For native-tls, we either need to add API or also expose TlsConnector directly.