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

`swarm.otel` (what the swarm collector is to every hive: its domain,
receiver ports, producer names, client id and the audiences that client
may present) moves to nix/host-modules/swarm-otel-service.nix, together
with the `journaldUnits` removal module and the helpers its defaults
read. Everything else -- the `deploy.swarm-otel` options, the whole
`config` block including `containers.swarm-otel`, and the helpers only
they read -- stays in nix/host-modules/swarm-otel.nix, which default.nix
now imports after the new file.

The service file needs `domainBase` (for `domain`), `baoCfg` (for
`storeProducerName`) and `cfg` plus `pushAudiences` (for `audience`).
`swarmDomain` and `domainBase` move. `cfg`, `hyperhiveCfg`, `baoCfg`,
`vmCfg`, `vlCfg`, `metricsPushUrl`, `logsPushUrl` and `pushAudiences`
are read on both sides and none is an option, so each is bound in both
files; the comments on the push URLs say the two copies must agree.
`swarmCfg` was bound and never read, so neither file carries it.

A pure move: option paths, option definitions and config are unchanged.
Comments changed: the transition comment above `deploy.swarm-otel` now
names the file `swarm.otel` lives in, and the push-URL and
`pushAudiences` comments now describe the per-file readers and the
duplicate. Three otel fixtures evaluate to the same host and container
toplevel derivations, `swarm.otel.*` and `deploy.swarm-otel.*` values
before and after.

Refs #3742

nix/host-modules/default.nix

@@ -52,6 +52,7 @@
     ./swarm-controller.nix
     ./swarm-grafana-service.nix
     ./swarm-grafana.nix
+    ./swarm-otel-service.nix
     ./swarm-otel.nix
     ./swarm-snapshot-store.nix
     ./swarm-ui.nix