Report progress for extra data downloads, as we do for OSTree pulls

Otherwise, clients such as GNOME Software won't be able to report
any progress once the flatpak application has been downloaded and
we enter the stage to download the extra data.
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f