swarm-nats-auth: answer callout requests concurrently, skip expired ones

Each request now runs in its own task, at most CONCURRENT_REQUESTS (32)
at once, instead of being awaited inline by the subscription loop. A
request whose exp (nats-server sets it to send time plus its
authorization.timeout) has passed is skipped without a lookup or reply.

Fixes #5434.

swarm-nats-auth/src/oidc_secret.rs

@@ -87,7 +87,7 @@ impl OidcSecret {
         }))
     }
 
-    fn new(secret: StoredSecret) -> Self {
+    pub(crate) fn new(secret: StoredSecret) -> Self {
         Self { secret }
     }