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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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

@@ -0,0 +1,20 @@
+package resolver
+
+import (
+	"net/netip"
+)
+
+func uniqueIPsToSlice(uniqueIPs map[string]struct{}) (ips []netip.Addr) {
+	ips = make([]netip.Addr, 0, len(uniqueIPs))
+	for key := range uniqueIPs {
+		ip, err := netip.ParseAddr(key)
+		if err != nil {
+			panic(err)
+		}
+		if ip.Is4In6() {
+			ip = netip.AddrFrom4(ip.As4())
+		}
+		ips = append(ips, ip)
+	}
+	return ips
+}