Implement a "fixed" sensor

[TheTinkerDad]

Sometimes it makes sense to return a fixed value to Home Assistant - this could be for something like a debug sensor or a configuration that doesn't really change on the monitored device.

It should be something like

    - name: My Fixed Sensor
      kind: fixed
      sensorid: myfixedsensor
      script: ""
      unitofmeasurement: ""
      icon: mdi:wrench-check
      value: Whatever You Want