Count downloads from content delivery network

[Following my comment here](https://github.com/haskell/hackage-server/issues/1342#issuecomment-5280844764), we can get reliable download counts despite the existence of a CDN via a redirection scheme:

* [Add the Cache-Control: no-store](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Cache-Control) header for the download endpoint;
* Log the download statistic.
* Return HTTP302 (Redirect) to another endpoint with the current CDN caching configuration.

I am happy to implement this feature.

One point that is unclear is whether this issue should be address _after_ https://github.com/haskell/hackage-server/issues/1342, given that the server will necessarily process more download counts.

Start by reviewing the download endpoint and the linked discussion in issue #1342, then verify how the current CDN caching configuration is applied. Confirm how to add Cache-Control: no-store, record the download statistic, and return an HTTP 302 to the CDN-backed endpoint. Done means reliable counts without disrupting CDN delivery, with the dependency on issue #1342 resolved.

Written by the indexing model from the issue text.