swarm-nats: take the callout issuer and user keys from the store

`swarm-bao-nats-callout-xkey` runs `ensure-callout-keys`, importing
/var/lib/swarm-nats-callout/{issuer,callout-user}.seed when present so the
keys keep their public halves, then fetches all three public halves from
the store. The verb removes each seed file once a read of the store returns
a seed; when the store's seed differs, it is kept and a warning names the
file. No tmpfiles rule touches those files, since one could run before the
import with the store sealed and leave the next run to mint a new key.

The server renders the issuer and user public halves from the fetched files,
checked for shape as the xkey's is, and does not start without them. The
responder reads both seeds from the store, so the copies into the container
and their LoadCredential entries are gone, and a tmpfiles rule removes the
copies a container still holds. `swarm-nats-callout-keys` is removed.

On the store's host the fetch unit is also ordered after
`swarm-bao-secret-publisher-policy`, so the publisher's denial of the seed
paths is written before the unit creates or keeps a seed there.

`deploy.nats.autoGenerateCallout`, `calloutUserPublicKey`,
`calloutIssuerPublicKey`, `calloutUserSeedFile` and `calloutIssuerSeedFile`
are removed with `mkRemovedOptionModule`, under `swarm.nats` too, each
message saying the keys live in the store.

nix/module-eval/nats-tls.nix

@@ -43,7 +43,6 @@ let
   # The queue on a host whose store is elsewhere: no local policy unit.
   queueNoStore = hive {
     deploy.nats.enable = true;
-    deploy.nats.autoGenerateCallout = true;
   };
 
   # The store's host with the hive's own reader pointed somewhere else.