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/configuration/settings/settings.go

@@ -132,7 +132,7 @@ func (s *Settings) SetDefaults() {
 	s.IPv6.setDefaults()
 	s.PublicIP.setDefaults()
 	s.Shadowsocks.setDefaults()
-	s.Storage.setDefaults()
+	s.Storage.SetDefaults()
 	s.System.setDefaults()
 	s.Version.setDefaults()
 	s.VPN.setDefaults()
@@ -213,7 +213,7 @@ func (s *Settings) Read(r *reader.Reader, warner Warner) (err error) {
 			return s.PublicIP.read(r, warner)
 		},
 		"shadowsocks": s.Shadowsocks.read,
-		"storage":     s.Storage.read,
+		"storage":     s.Storage.Read,
 		"system":      s.System.read,
 		"updater":     s.Updater.read,
 		"version":     s.Version.read,