add standalone clightning-rest service

- Rename `services.rtl.cl-rest` to `services.clightning-rest`.
  `clightning-rest` is generally useful for connecting external REST clients
  to clightning.

- Add a dedicated network namespace in netns-isolation.

- Add nodeinfo entry.

- Add datadir (which contains REST auth data) to backups.

modules/modules.nix

@@ -12,6 +12,7 @@
     ./bitcoind.nix
     ./clightning.nix
     ./clightning-plugins
+    ./clightning-rest.nix
     ./spark-wallet.nix
     ./lnd.nix
     ./lnd-rest-onion-service.nix # Requires onion-addresses.nix