Added Custom Fields on Shipping Methods

We added support for Custom Fields on Shipping Methods. This allows you to view and edit the values of a Shipping Methods Custom Fields if those have been added using the API.

Furthermore, we redesigned the Shipping Methods and the Taxes tab in the Merchant Center to align the user experience with that of the other tabs. You can now sort the Shipping Methods and Tax Categories in the list by name, by date of creation, or by date of last modification.