Add a `last_downloaded_at` field to the Package model

🔥 Problem

Download statistics for the package registry is something that keep coming back from users. Related issue: #293739.

🚒 Solution

This issue is to propose a simple first iteration: have a last_downloaded_at field that is updated each time a package is pulled from the registry.

Why such simple approach?

1. It could fill some of the user needs.
2. Cleanup policies could leverage this field to have rules like:
   • Remove any package that has not been downloaded in the last 30 days.
   • Remove any package that was created 6 months ago and was never downloaded.