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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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/extract.go

@@ -0,0 +1,37 @@
+package unzip
+
+import (
+	"archive/zip"
+	"bytes"
+	"io"
+	"path/filepath"
+	"strings"
+)
+
+func zipExtractAll(zipBytes []byte) (contents map[string][]byte, err error) {
+	r, err := zip.NewReader(bytes.NewReader(zipBytes), int64(len(zipBytes)))
+	if err != nil {
+		return nil, err
+	}
+	contents = map[string][]byte{}
+	for _, zf := range r.File {
+		fileName := filepath.Base(zf.Name)
+		if !strings.HasSuffix(fileName, ".ovpn") &&
+			!strings.HasSuffix(fileName, ".conf") {
+			continue
+		}
+		f, err := zf.Open()
+		if err != nil {
+			return nil, err
+		}
+		defer f.Close()
+		contents[fileName], err = io.ReadAll(f)
+		if err != nil {
+			return nil, err
+		}
+		if err := f.Close(); err != nil {
+			return nil, err
+		}
+	}
+	return contents, nil
+}