openssl support

[Schira4396]

If the target is an older version of sql server, it is possible that it only supports tls 1.0. Although it is not secure, can the openssl feature be added to enable tiberius to connect to it, thereby increasing compatibility？

For example, the target is Windows Server 2008 and mandatory encryption is enabled.

If the rust-openssl library is used, then it is possible to:

`let mut ctx: openssl::ssl::SslConnectorBuilder = SslConnector::builder(SslMethod::tls())?;
ctx.set_security_level(0);
ctx.set_options(SslOptions::ALLOW_UNSAFE_LEGACY_RENEGOTIATION);
ctx.set_verify(openssl::ssl::SslVerifyMode::NONE);
ctx.set_options(SslOptions::DONT_INSERT_EMPTY_FRAGMENTS);`

[Schira4396]

It's over. Just modify opentls and add relevant configurations. However, it is still recommended to add relevant function interfaces. Thank you, author!

pub fn build(&self) -> crate::Result<TlsConnector> {
    static ONCE: Once = Once::new();
    ONCE.call_once(openssl_probe::init_ssl_cert_env_vars);

    let mut connector = SslConnector::builder(SslMethod::tls())?;
    // add
    connector.set_security_level(0);
    connector.set_options(SslOptions::ALLOW_UNSAFE_LEGACY_RENEGOTIATION);
    connector.set_verify(openssl::ssl::SslVerifyMode::NONE);
    connector.set_options(SslOptions::DONT_INSERT_EMPTY_FRAGMENTS);
    if let Some(ref identity) = self.identity {
        connector.set_certificate(&identity.cert)?;
        connector.set_private_key(&identity.pkey)?;

        for cert in identity.chain.iter().rev() {
            connector.add_extra_chain_cert(cert.to_owned())?;
        }
    }

    supported_protocols(self.min_protocol, self.max_protocol, &mut connector)?;

    if self.disable_built_in_roots {
        connector.set_cert_store(X509StoreBuilder::new()?.build());
    }

    for cert in &self.root_certificates {
        if let Err(err) = connector.cert_store_mut().add_cert(cert.0.clone()) {
            debug!("add_cert error: {:?}", err);
        }
    }

    #[cfg(target_os = "android")]
    crate::load_android_root_certs(&mut connector)?;

    Ok(TlsConnector {
        connector: connector.build(),
        use_sni: self.use_sni,
        accept_invalid_hostnames: self.accept_invalid_hostnames,
        accept_invalid_certs: self.accept_invalid_certs,
    })
}