gbmcweb: Add StandbyOffline feature in meson options

Adds a new build option `enable-standby-offline` to enable StandbyOffline state for sensors and make it

Tested:
Verified on a sensor that went missing:
```
  "Status": {
    "Health": "Critical",
    "State": "UnavailableOffline"
  },
```
PiperOrigin-RevId: 843491942
Change-Id: Ice3057614c854f6f3f139a2e344392f7702a50b8