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/vpn/interfaces.go

@@ -45,8 +45,6 @@ type Provider interface {
 	GetConnection(selection settings.ServerSelection, ipv6Supported bool) (connection models.Connection, err error)
 	OpenVPNConfig(connection models.Connection, settings settings.OpenVPN, ipv6Supported bool) (lines []string)
 	Name() string
-	FetchServers(ctx context.Context, minServers int) (
-		servers []models.Server, err error)
 }
 
 type PortForwarder interface {