server: Add new server.client_introduction config block. (#26315)

The new configuration block exposes some key options which allow
cluster administrators to control certain client introduction
behaviours.

This change introduces the new block and plumbing, so that it is
exposed in the Nomad server for consumption via internal processes.

command/agent/testdata/basic.hcl

@@ -168,6 +168,12 @@ server {
     }
   }
 
+  client_introduction {
+    enforcement          = "warn"
+    default_identity_ttl = "5m"
+    max_identity_ttl     = "30m"
+  }
+
   license_path = "/tmp/nomad.hclic"
 }