nix: split swarm-nats into service and deploy-mode files

`swarm.nats` (what the queue is to every hive: domain, ports, client id)
moves to nix/host-modules/swarm-nats-service.nix, together with the only
two helpers it reads, `swarmDomain` and `domainBase`. Everything else --
the `deploy.nats` options, the whole `config` block including
`containers.swarm-nats`, and the helpers only they read -- stays in
nix/host-modules/swarm-nats.nix, which default.nix now imports alongside
the new file.

A pure move: option paths, option definitions and config are unchanged
apart from the transition comment above `deploy.nats`, which now names the
file `swarm.nats` lives in. The nats fixtures evaluate to the same host and
container toplevel derivations before and after.

Refs #3742

nix/host-modules/default.nix

@@ -44,6 +44,7 @@
     ./swarm-bao.nix
     ./swarm-ca.nix
     ./swarm-secret-publisher.nix
+    ./swarm-nats-service.nix
     ./swarm-nats.nix
     ./swarm-controller.nix
     ./swarm-grafana.nix