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.

README.md

@@ -73,6 +73,7 @@ NixOS modules ([src](modules/modules.nix))
     * [rebalance](https://github.com/lightningd/plugins/tree/master/rebalance): keeps your channels balanced
     * [summary](https://github.com/lightningd/plugins/tree/master/summary): print a nice summary of the node status
     * [zmq](https://github.com/lightningd/plugins/tree/master/zmq): publishes notifications via ZeroMQ to configured endpoints
+  * [clightning-rest](https://github.com/Ride-The-Lightning/c-lightning-REST): REST server for clightning
   * [lnd](https://github.com/lightningnetwork/lnd) with support for announcing an onion service and [static channel backups](https://github.com/lightningnetwork/lnd/blob/master/docs/recovery.md)
     * [Lightning Loop](https://github.com/lightninglabs/loop)
     * [Lightning Pool](https://github.com/lightninglabs/pool)