Add convenience function set_if on ClientConfig

[theelderbeever]

Adds a simple function ClientConfig.set_if that wraps ClientConfig.set to allow for passing values that are options. This is helpful for configuring clients dynamically via config files that may or may not have values set.

// Such a value could be read from a config file if the user wanted to set it otherwise, it would read in as `None`.
let sasl_mechanism = Some("PLAINTEXT");

let config = ClientConfig::new()
    .set_if(
        "sasl.mechanism", sasl_mechanism,
    );