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

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

`hyperhiveCfg` (an alias for `config.services.hyperhive`, not an option) is
read by both halves, so it is duplicated into the service file rather than
shared.

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

Refs #3742

nix/host-modules/default.nix

@@ -54,6 +54,7 @@
     ./swarm-snapshot-store.nix
     ./swarm-ui.nix
     ./swarm-victorialogs.nix
+    ./swarm-victoriametrics-service.nix
     ./swarm-victoriametrics.nix
     ./swarm-wireguard.nix
     ./swarm.nix