Implement ubus system uptime sensor

[denisfrench]

I note the discussion around creating sensors here #9 , and also that commit aaddd2b implemented capture of the openwrt.exec service call stdout return on the event bus.

The resulting event from calling the /usr/bin/uptime command example from the readme is:

event_type: openwrt_exec_result
data:
  address: 192.168.1.1
  id: Ntgr-X4S-R7800
  command: /usr/bin/uptime
  code: 0
  stdout: |2
     09:50:44 up 3 days, 16:23,  load average: 0.21, 0.19, 0.16
origin: LOCAL
time_fired: "2023-08-18T23:50:44.891336+00:00"
context:
  id: 01H85GXXWV1VG59SAMTENB3HBH
  parent_id: null
  user_id: null

For those with a moderate amount of Home Assistant knowledge, it should therefore be possible to setup an automation to periodically call the openwrt.exec service (https://www.home-assistant.io/docs/automation/trigger/#time-pattern-trigger), and capture the output within a template trigger sensor (https://www.home-assistant.io/integrations/template/#trigger-based-template-binary-sensors-buttons-images-numbers-selects-and-sensors). I haven't tried it yet; capturing and reformatting the output looks a little tricky, but doable.

However I have confirmed that the terminal command ubus call system info | jsonfilter -e "@['uptime']" directly returns OpenWRT system uptime in seconds, and suggest that implementing this as a sensor within the integration is a more elegant and user friendly solution.

Thank you for your work on this in any case; it is already very useful in monitoring OpenWRT connectivity within Home Assistant.