[REST] Add uptime counter to systeminfo endpoint

[spacemanspiff2007]

Handling startup of openHAB from a HABApp perspective is hard.
OpenHAB might be stuck at startLevel 70 and never reach startLevel 100 if thing configs are broken.
Even if openHAB reaches startLevel 100 it's possible that the item state restore event (e.g. from MapDB) is queued up so that the item changes to the restored state significantly after openHAB reached startLevel 100 (I've seen up to 15s).

It would be very helpful if there was an uptime field in systemInfo that would contain the openHAB uptime e.g. in secs.
That way HABApp could easily differentiate e.g. between a openHAB restart and a HABApp restart.

[spacemanspiff2007]

@J-N-K Would you mind taking a look at this? It would be tremendously helpful to sync the startup with HABApp if there was a field that would contain the openHAB system uptime e.g. as an integer and it should be a very tiny code change.

e.g. like this:

{
  "version": "5",
  "locale": "de_DE",
  "measurementSystem": "SI",
  "uptime": 355,     <-------------- Uptime in seconds, here 5 mins and 55 secs
  "runtimeInfo": {
    "version": "4.0.0",
    "buildString": "Release Build"
  },
  "links": [
    ...
  ]
}

An alternative could also be to just add the time started as an ISO timestamp. (e.g. 2023-09-06T12:02:22+00:00)

[I hope it's okay to ping you - if not please let me know and I will refrain from pinging you for feature requests in the future]

[spacemanspiff2007]

Thank you very much for your help!