Allow Home Assistant OTA Update entity to show progress while updating

[jbouwh]

Background

When and OTA update is in progress, no progress percentage is shown in Home Assistant.
The current MQTT OTA config update set json_attributes which should not be used.

Proposed change

This PR will use the value_template to extract latest_version, installed_version and update_percentage from state_topic instead. This will allow to use the progress attribute to indicate the progress percentage in Home Assistant.
There is no need to used latest_version_topic and latest_version_template.

A sample config payload would look like:

{
  "availability": [
    {
      "topic": "zigbee2mqtt/bridge/state",
      "value_template": "{{ value_json.state }}"
    }
  ],
  "command_topic": "zigbee2mqtt/bridge/request/device/ota_update/update",
  "device": {
    "hw_version": 1,
    "identifiers": [
      "zigbee2mqtt_0x943469fffec8bf65"
    ],
    "manufacturer": "IKEA",
    "model": "STYRBAR remote control",
    "model_id": "E2001/E2002",
    "name": "Slaapkamer wandbediening",
    "sw_version": "2.4.5",
    "via_device": "zigbee2mqtt_bridge_0xe0798dfffe1a1fc9"
  },
  "device_class": "firmware",
  "entity_category": "config",
  "entity_picture": "https://github.com/Koenkk/zigbee2mqtt/raw/master/images/logo.png",
  "name": null,
  "object_id": "slaapkamer_steven_wandbediening",
  "origin": {
    "name": "Zigbee2MQTT",
    "sw": "2.0.0",
    "url": "https://www.zigbee2mqtt.io"
  },
  "payload_install": "{\"id\": \"0x943469fffec8bf65\"}",
  "state_topic": "zigbee2mqtt/Slaapkamer wandbediening",
  "unique_id": "0x943469fffec8bf65_update_zigbee2mqtt",
  "value_template": "{\"latest_version\": \"{{ value_json['update']['latest_version'] }}\", \"installed_version\": \"{{ value_json['update']['installed_version'] }}\", \"update_percentage\": {{ value_json['update'].get('progress', 'null') }} }"
}

[Koenkk]

Thanks! ❤️