feat(storage): storage file structure changes (#3301)

- migrate persisted server data storage from `/gluetun/servers.json` to `/gluetun/servers/`
- add `STORAGE_SERVERS_ENABLED=on` to enable or disable on-disk server data storage
- add `STORAGE_SERVERS_DIRECTORY_PATH=/gluetun/servers` to configure where per-provider server files are stored
- keep backward compatibility with legacy `STORAGE_FILEPATH=/gluetun/servers.json`
- automatically read and migrate legacy `/gluetun/servers.json` into the new `/gluetun/servers/` layout when needed
- try to remove the legacy servers file after a successful migration to the new storage directory
- switch persisted server data from one large JSON file to a manifest plus per-provider JSON files
- add `UPDATER_PREFER_DIRECT_DOWNLOAD` to allow preferring direct download of provider server data
- keep deprecated updater flags `-enduser` and `-maintainer` as no-op warnings for backward compatibility
- preserve compatibility checks so persisted server data is discarded when its schema version no longer matches the built-in data
- allow preferred persisted provider data to override built-in data when versions match
- servers data now lives at https://github.com/qdm12/gluetun-servers/tree/main/pkg/servers

internal/updater/loop/loop.go

@@ -50,7 +50,7 @@ func NewLoop(settings settings.Updater, providers updater.Providers,
 			status:   constants.Stopped,
 			settings: settings,
 		},
-		updater:      updater.New(client, storage, providers, logger),
+		updater:      updater.New(client, storage, providers, logger, *settings.PreferDirectDownload),
 		logger:       logger,
 		start:        make(chan struct{}),
 		running:      make(chan models.LoopStatus),