refactor(storage): new storage file structure

- new directory structure containing manifest.json and one json file per provider, by default.
- the manifest.json file can specify a filepath for each vpn provider
- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch
- `STORAGE_SERVERS_DIRECTORY_PATH` replaces `STORAGE_FILEPATH` (which is now a migration source only). It sets the directory where server manifest and per-provider JSON files are stored (default: `/gluetun/servers/`).
- First-run migration: On startup, gluetun checks for the old /gluetun/servers.json file; if found and no new manifest exists, it automatically migrates all data to /gluetun/servers/ directory structure
- Silent fallback: If legacy file isn't found, uses the new directory path normally
- Legacy cleanup: After successful migration, attempts to remove the old fat JSON file (logs warning only if removal fails, e.g., read-only bind mounts)

gluetun-servers/pkg/updaters/unzip/fetch.go

@@ -0,0 +1,40 @@
+package unzip
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+)
+
+func (u *Unzipper) FetchAndExtract(ctx context.Context, url string) (
+	contents map[string][]byte, err error,
+) {
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+	request.Header.Set("User-Agent", "gluetun")
+
+	response, err := u.client.Do(request)
+	if err != nil {
+		return nil, err
+	}
+	defer response.Body.Close()
+
+	if response.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("HTTP status code not OK: %s: %d %s",
+			url, response.StatusCode, response.Status)
+	}
+
+	b, err := io.ReadAll(response.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := response.Body.Close(); err != nil {
+		return nil, err
+	}
+
+	return zipExtractAll(b)
+}