Add `PolywrapClientBuilder`

[cbrzn]

Instead of:

let mut config = PolywrapClientConfig::new();
config.add(SystemClientConfig::default().into());
let config = config.build();
let client = PolywrapClient::new(config);

We do:

let client = PolywrapClientBuilder::default()
    .add(SystemClientConfig::default().into())
    .build();