#tlbmc Record BootTimeMonitor checkpoints during root Chassis.Reset

Add BootTimeMonito checkpoint support to tlBMC when triggering a root
Chassis.Reset (`PowerCycle`) via Redfish:
1. Add `IsBootTimeMonitorEnabled()`, `SetBootTimeNodeCheckpoint()`, and
   `SetBootTimePsuCheckpoint()` to `Store`, `StoreImpl`, and `MockStore`.
   When enabled, `StoreImpl::Create` registers the `"bmc"` node (`kBootTimeTagBMC`)
   and `"host0" .. "host{N-1}"` nodes (`kBootTimeTagHost`) with `BoottimeApi`.
2. Update `TriggerDefaultRootChassisReset` in `chassis.cc` to record the
   `"RebootStart"` checkpoint for `"bmc"` at the start of root `PowerCycle` and
   broadcast the `"TrayPowercycle"` PSU checkpoint prior to asserting the
   hotswap GPIO.

Tested:
```
{
  "@odata.id": "/redfish/v1/Managers/bmc/Oem/Google/BootTime",
  "@odata.type": "#GoogleBootTime.v1_0_0.GoogleBootTime",
  "Breakdown": [],
  "Durations": [
    { "Duration": 0.0, "Name": "Firmware" },
    { "Duration": 0.0, "Name": "Loader" },
    { "Duration": 7.715, "Name": "Kernel" },
    { "Duration": 0.0, "Name": "InitRD" },
    { "Duration": 155.288, "Name": "Userspace" }
  ],
  "Id": "BootTimeData",
  "Name": "Boot time data",
  "RebootFlow": [
    {
      "MonotonicStartTime": 93374180,
      "Stage": "RebootStart",
      "StartTime": "2026-09-22T08:20:03.379+00:00"
    },
    {
      "MonotonicStartTime": 93374240,
      "Stage": "TrayPowercycle",
      "StartTime": "2026-09-22T08:20:03.436+00:00"
    },
    {
      "MonotonicStartTime": 167210,
      "Stage": "UserspaceEnd",
      "StartTime": "2026-09-22T08:23:19.710+00:00"
    },
    {
      "MonotonicStartTime": 167210,
      "Stage": "RebootEnd",
      "StartTime": "2026-09-22T08:23:19.711+00:00"
    }
  ],
  "Stat": {
    "IsRebooting": false,
    "LoaderReadyTimestamp": null,
    "PowerSource": "AC",
    "UpTime": 38970,
    "UpTimestamp": 1790065399711
  },
  "Status": {
    "Health": "Ok",
    "State": "Enabled"
  },
  "Total": 196.332
}

```

Google-Bug-Id:395219125
PiperOrigin-RevId: 992087795
Change-Id: I1fcfdc3a594845ee30a2ed211d2f978e6941f3cf