offload_lfc_interval_seconds in ComputeSpec (#12447)

- Add ComputeSpec flag `offload_lfc_interval_seconds` controlling
  whether LFC should be offloaded to endpoint storage. Default value
  (None) means "don't offload".
- Add glue code around it for `neon_local` and integration tests.
- Add `autoprewarm` mode for `test_lfc_prewarm` testing
  `offload_lfc_interval_seconds` and `autoprewarm` flags in conjunction.
- Rename `compute_ctl_lfc_prewarm_requests_total` and
`compute_ctl_lfc_offload_requests_total` to
`compute_ctl_lfc_prewarms_total`
  and `compute_ctl_lfc_offloads_total` to reflect we count prewarms and
  offloads, not `compute_ctl` requests of those.
  Don't count request in metrics if there is a prewarm/offload already
  ongoing.

https://github.com/neondatabase/cloud/issues/19011
Resolves: https://github.com/neondatabase/cloud/issues/30770

libs/compute_api/tests/cluster_spec.json

@@ -90,6 +90,11 @@
                 "value": "off",
                 "vartype": "bool"
             },
+            {
+                "name": "offload_lfc_interval_seconds",
+                "value": "20",
+                "vartype": "integer"
+            },
             {
                 "name": "neon.safekeepers",
                 "value": "127.0.0.1:6502,127.0.0.1:6503,127.0.0.1:6501",