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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,15 @@
+package protonvpn
+
+import "strings"
+
+func codeToCountry(countryCode string, countryCodes map[string]string) (
+	country string, warning string,
+) {
+	countryCode = strings.ToLower(countryCode)
+	country, ok := countryCodes[countryCode]
+	if !ok {
+		warning = "unknown country code: " + countryCode
+		country = countryCode
+	}
+	return country, warning
+}