This PR adds structured logging for the aggregator service. Currently, only the text handler is used, but I can extend this to use the JSON handler as well. In this case, probably some code that is shared between the aggregator and the downloader would need to be moved to a common package.
I was also wondering, whether this repo is moving to Go 1.21 at the future, since `slog` was introduced in to the standard lib in 1.21. So currently, this still relies on the `x/exp` package.
Fixes #462
* Remove verbose flag from downloader.
* Do structured http logging in forwarder, too.
* Use structured logging to separate http traffic of downloader from forwarder.
