Format mqtt json fields

I suggest that a json_attribute_templates: is added that can be used the following way

- platform: mqtt
  name: "Peter train from work"
  state_topic: "trafikverket/peter_train_from_work"
  value_template: "{{ value_json.state }}"
  payload_available: "online"
  payload_not_available: "offline"
    id: "{{ }}"
    canceled: "{{ value_json.canceled | bool }}"
    advertised_time_at_location: "{{ value_json.advertisedTimeAtLocation | datetime }}"
    estimated_time_at_location: "{{ value_json.estimatedTimeAtLocation | datetime }}"
    time_at_location: "{{ value_json.timeAtLocation | datetime }}"
    other_information: "{{ value_json.otherInformation }}"
    deviations: "{{ value_json.deviations }}"
    modified_time: "{{ value_json.modifiedTime | datetime }}"

This way you can change the field names to use the snake_case used in Home assistant even if the mqtt json uses javascripts camelCase standard, also fields can be converted to the right type!

Added a pull request to add this functionality!