Add format setter and getter to CarrierService resource.

@pickle27 for review

## Problem

When setting the format on a CarrierService resource, the format will get set as an attribute rather than an attribute since active resource has a format class property.

This problem was reported in the forums https://ecommerce.shopify.com/c/shopify-apis-and-technology/t/overlapping-format-attribute-in-shopify_python_api-203749

## Solution

Webhooks also have a format attribute, but also have format property to make sure the format gets set on the attribute instead, so I did the same for CarrierService.