Add customization options when using native-tls

[gordonwang0]

We are using rumqttc with native-tls and require the option to configure multiple or no CAs and client certificate chains. Right now, this is only possible when using rustls.

The TlsConfiguration struct only supports customization options when using rustls:

rumqtt/rumqttc/src/lib.rs

Line 340 in b845a5b

Rustls(Arc<ClientConfig>),

We would like to have the equivalent functionality for native-tls. Ideally, the Native variant would take a native_tls::TlsConnectorBuilder to allow more customization of the TLS connection:

// Proposed `Native` variant
Native(native_tls::TlsConnectorBuilder)

The native_tls::TlsConnectorBuilder should then be used to construct the TLS connection.