Add testing_migration Parameter Support for Reports

[jghansell]

With the modernization of the QBO Reports API, testing the changes in development will require adding the testing_migration parameter to requests, as mentioned in this medium post from Intuit.

https://medium.com/intuitdev/upcoming-changes-to-reports-apis-5083ec9aadce

There is no easy way to add this to a report request (that I was able to find), besides adding a custom request Interceptor that adds it to the URI. Adding explicit support for this parameter, at least until the changes go live on July 1 2026, would make testing these new changes easier.

I am new to the internals of the codebase, but I believe this change would only require the following:

1. Adding a new entry for testing_migration to RequestElements
2. Adding that entry to PrepareRequestInterceptor.isKeyValueExpected
3. Adding a private field + public getter/setter to ReportService
4. Updating ReportService.prepareReport to add that field to the intuitMessage