Add support for "merchant_id" shipment option

[annatarviola]

Feature Request Is New

• I have verified that the requested feature does not already exist or has not already been requested.

Description of the feature

The merchant_id shipment option is required for shipping with the DHL eCommerce Solutions by Default carrier account (see in DHL eCommerce (Default) International Shipping Guide)

It looks like this option needs to be added here so that C# library users can use the DHL eCS default account: https://github.com/EasyPost/easypost-csharp/blob/master/EasyPost/Models/API/Options.cs

[nwithan8]

Thanks for letting us know, we'll add it in.

In the meantime, you can use the AddAdditionalOption function to add an option key-value pair that may not be explicitly supported.