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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,67 @@
+package openvpn
+
+import (
+	"context"
+	"net/http"
+)
+
+// FetchMultiFiles fetches multiple Openvpn files in parallel and
+// parses them to extract each of their host. A mapping from host to
+// URL is returned.
+func FetchMultiFiles(ctx context.Context, client *http.Client, urls []string,
+	failEarly bool,
+) (hostToURL map[string]string, errors []error) {
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+
+	hostToURL = make(map[string]string, len(urls))
+
+	type Result struct {
+		url  string
+		host string
+	}
+
+	results := make(chan Result)
+	defer close(results)
+	errorsCh := make(chan error)
+	defer close(errorsCh)
+
+	for _, url := range urls {
+		go func(url string) {
+			host, err := FetchFile(ctx, client, url)
+			if err != nil {
+				errorsCh <- err
+				return
+			}
+			results <- Result{
+				url:  url,
+				host: host,
+			}
+		}(url)
+	}
+
+	for range urls {
+		select {
+		case result := <-results:
+			hostToURL[result.host] = result.url
+		case err := <-errorsCh:
+			if !failEarly {
+				errors = append(errors, err)
+				break
+			}
+
+			if len(errors) == 0 {
+				errors = []error{err} // keep only the first error
+				// stop other operations, this will trigger other errors we ignore
+				cancel()
+			}
+		}
+	}
+
+	if len(errors) > 0 && failEarly {
+		// we don't care about the result found
+		return nil, errors
+	}
+
+	return hostToURL, errors
+}