Add support for checkoutShippingLineUpdate mutation

[rohailaltaf]

Adds support for the checkoutShippingLineUpdate mutation so you can select shipping method programmatically.

Example usage:

const checkoutId = 'Z2lkOi8vc2hvcGlmeS9DaGVja291dC9kMTZmM2EzMDM4Yjc4N=';
const shippingRateHandle = 'Standard';

client.checkout.updateShippingLine(checkoutId, shippingRateHandle).then(checkout => {
    // Do something with the updated checkout
});

[rohailaltaf]

Signed the CLA but I dont think I have permissions to re-run the checks 😬