Improve progress report calculation

Now that we have access to the total extra-data download size,
we can have much more precise progress reports by summing up
all the download sizes from OSTree and extra-data.

This patch makes the progress report callback use the extra-data
sizes, as well as calculate the average size (in bytes) of the
content.

flatpak/flatpak#609
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f