[Shipping labels] Add yosemite support for label purchase

[rachelmcr]

We need to add support in WooShippingAction and WooShippingStore for a label purchase.

We don't need to store the response; we'll use it to poll for the label purchase status and then return the purchased label directly. See ShippingLabelStore for an example.

For now, we can stub out any logic around polling the status of the label purchase. This will be handled as part of #14053.

[dangermattic]

Thanks for reporting! 👍