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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,57 @@
+package example
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+)
+
+type apiData struct {
+	Servers []apiServer `json:"servers"`
+}
+
+type apiServer struct {
+	OpenVPNHostname   string `json:"openvpn_hostname"`
+	WireguardHostname string `json:"wireguard_hostname"`
+	Country           string `json:"country"`
+	Region            string `json:"region"`
+	City              string `json:"city"`
+	WgPubKey          string `json:"wg_public_key"`
+}
+
+func fetchAPI(ctx context.Context, client *http.Client) (
+	data apiData, err error,
+) {
+	// TODO: adapt this URL and the structures above to match the real
+	// API models you have.
+	const url = "https://example.com/servers"
+
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return data, err
+	}
+
+	response, err := client.Do(request)
+	if err != nil {
+		return data, err
+	}
+
+	if response.StatusCode != http.StatusOK {
+		_ = response.Body.Close()
+		return data, fmt.Errorf("HTTP status code not OK: %d %s",
+			response.StatusCode, response.Status)
+	}
+
+	decoder := json.NewDecoder(response.Body)
+	if err := decoder.Decode(&data); err != nil {
+		_ = response.Body.Close()
+		return data, fmt.Errorf("decoding response body: %w", err)
+	}
+
+	if err := response.Body.Close(); err != nil {
+		return data, fmt.Errorf("closing response body: %w", err)
+	}
+
+	return data, nil
+}