feat(storage): storage file structure changes (#3301)

- migrate persisted server data storage from `/gluetun/servers.json` to `/gluetun/servers/`
- add `STORAGE_SERVERS_ENABLED=on` to enable or disable on-disk server data storage
- add `STORAGE_SERVERS_DIRECTORY_PATH=/gluetun/servers` to configure where per-provider server files are stored
- keep backward compatibility with legacy `STORAGE_FILEPATH=/gluetun/servers.json`
- automatically read and migrate legacy `/gluetun/servers.json` into the new `/gluetun/servers/` layout when needed
- try to remove the legacy servers file after a successful migration to the new storage directory
- switch persisted server data from one large JSON file to a manifest plus per-provider JSON files
- add `UPDATER_PREFER_DIRECT_DOWNLOAD` to allow preferring direct download of provider server data
- keep deprecated updater flags `-enduser` and `-maintainer` as no-op warnings for backward compatibility
- preserve compatibility checks so persisted server data is discarded when its schema version no longer matches the built-in data
- allow preferred persisted provider data to override built-in data when versions match
- servers data now lives at https://github.com/qdm12/gluetun-servers/tree/main/pkg/servers

Dockerfile

@@ -249,6 +249,7 @@ ENV VPN_SERVICE_PROVIDER=pia \
     UPDATER_PERIOD=0 \
     UPDATER_MIN_RATIO=0.8 \
     UPDATER_VPN_SERVICE_PROVIDERS= \
+    UPDATER_PREFER_DIRECT_DOWNLOAD=no \
     UPDATER_PROTONVPN_EMAIL= \
     UPDATER_PROTONVPN_PASSWORD= \
     # Public IP
@@ -257,7 +258,8 @@ ENV VPN_SERVICE_PROVIDER=pia \
     PUBLICIP_API=ipinfo,ifconfigco,ip2location,cloudflare \
     PUBLICIP_API_TOKEN= \
     # Storage
-    STORAGE_FILEPATH=/gluetun/servers.json \
+    STORAGE_SERVERS_ENABLED=on \
+    STORAGE_SERVERS_DIRECTORY_PATH=/gluetun/servers/ \
     # Pprof
     PPROF_ENABLED=no \
     PPROF_BLOCK_PROFILE_RATE=0 \