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

compute_tools/tests/pg_helpers_tests.rs

@@ -31,6 +31,7 @@ mod pg_helpers_tests {
 wal_level = logical
 hot_standby = on
 autoprewarm = off
+offload_lfc_interval_seconds = 20
 neon.safekeepers = '127.0.0.1:6502,127.0.0.1:6503,127.0.0.1:6501'
 wal_log_hints = on
 log_connections = on