Support for Partner Center SDK

> In Azure/azure-sdk-for-go#3002 @jjmartin says:
> ### Feature Request
>
> I'd like to see support for the Partner Center SDK - specifically the Scenarios described here:
> https://docs.microsoft.com/en-us/partner-center/develop/manage-invoices
>
> (if there is already support, i'd like to see better documentation showing me how to get there)

After searching this repository for some of the operations and types defined in the docs linked, I didn't find a Swagger for this functionality.

Start with the Partner Center manage-invoices documentation linked in the issue, then search this repository for the listed operations and types and any existing Swagger or OpenAPI definitions. Determine whether the scenarios already have specifications; done means the requested Partner Center scenarios are represented here or the existing support is documented.

Written by the indexing model from the issue text.