Missing label_image_id for getting labels from rates #7

This was not documented so I added it to the OpenAPI XML spec and also added it to our C# client library. The issue in question is here:

kendallb/ShipEngineApi#1

You should pull my change over from my fork, but also have the documentation on the web site updated based on what support sent to the other developer?

https://www.shipengine.com/docs/labels/create-from-rate/