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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,43 @@
+package openvpn
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+)
+
+func FetchFile(ctx context.Context, client *http.Client, url string) (
+	host string, err error,
+) {
+	b, err := fetchData(ctx, client, url)
+	if err != nil {
+		return "", err
+	}
+
+	const rejectIP = true
+	const rejectDomain = false
+	hosts := extractRemoteHosts(b, rejectIP, rejectDomain)
+	if len(hosts) == 0 {
+		return "", fmt.Errorf("remote host not found for url %s", url)
+	}
+
+	return hosts[0], nil
+}
+
+func fetchData(ctx context.Context, client *http.Client, url string) (
+	b []byte, err error,
+) {
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	response, err := client.Do(request)
+	if err != nil {
+		return nil, err
+	}
+	defer response.Body.Close()
+
+	return io.ReadAll(response.Body)
+}