Support Reusable Workflows in Private Repositories

Describe the enhancement
Allow reusable workflows to be triggered even if they reside in a private repository. This is a documented limitation of reusable workflows, but it would be very helpful if it were changed.

I maintain microservices in my organisation’s private repos, and it would be good to be able to run integration tests on them by calling a single workflow residing in one of their repos. Currently this is not possible so I have to copy the testing workflow into each of the repos. Not very dry.