offline license utilization reporting (#25844)

Nomad Enterprise users operating in air-gapped or otherwise secured environments
don't want to send license reporting metrics directly from their
servers. Implement manual/offline reporting by periodically recording usage
metrics snapshots in the state store, and providing an API and CLI by which
cluster administrators can download the snapshot for review and out-of-band
transmission to HashiCorp.

This is the CE portion of the work required for implemention in the Enterprise
product. Nomad CE does not perform utilization reporting.

Ref: https://github.com/hashicorp/nomad-enterprise/pull/2673
Ref: https://hashicorp.atlassian.net/browse/NMD-68
Ref: https://go.hashi.co/rfc/nmd-210

command/agent/testdata/basic.hcl

@@ -348,8 +348,9 @@ reporting {
     enabled = true
   }
 
-  address         = "http://localhost:8080"
-  export_interval = "15m"
+  address                 = "http://localhost:8080"
+  export_interval         = "15m"
+  snapshot_retention_time = "24h"
 }
 
 keyring "awskms" {