Allow customers to stop `gh gei migrate-repo` after the archive is downloaded, so they can manually upload to blob storage

In some cases, when migrating from GitHub Enterprise Server or GitHub AE, a customer may want to upload their migration archive(s) to blob storage manually - for example because they want to use a custom blob storage solution or because they need to use special authentication to access blob storage.

We should add a way, when running `gh gei migrate-repo`, to generate and download the archives and then stop there, allowing the user to manually upload the archives to a destination of their choice.

Our existing behaviour should already allow the customer to pick up the migration where it left off and start the import, passing in the Git and metadata archive URLs.