This includes mirror selection, use of proxy, HTTP auth and posting of JSON for clean insights.
I manually tested various HTTP repositories with proxy and without. Using some or only .onion mirrors as well as swap repositories. Image downloading was migrated to the new library as well and is now respecting the proxy settings. Most of it is also using mirrors now.
The Downloader and HttpDownloader classes have been moved into the new library and transformed to Kotlin. Other classes extending Downloader remain in fdroidclient due to their dependencies and niche use-cases.
For convenience, a new tools/update-verification-metadata.sh is included.
This MR also upgrades the gradle version to 6.9.2.