ribbit.time_manager: Record and report boot time

[damz]

This PR records the initial boot time of the sensor (as defined as the first valid time we have acquired), and adds a new time_manager section to the aggregated sensor data:

"time_manager": {
  "has_valid_time": true,                     // Whether we have a valid time or not
  "source": "simulator",                      // The last source of the time ("unknown", "ntp", "gps", "simulator")
  "last_update": "2024-01-17T00:10:54Z",      // The last time the time was updated (or null if the time is unknown)
  "boot_time": "2024-01-17T00:10:54Z",        // The first time we recorded since the boot of the sensor
}

[keenanjohnson]

Looks good to me!