Mqtt_statestream : purpose complete attributes in json payload

Today in 2023.x release, mqtt_statestream only purpose to have attributes on individual flat list, and it is an incomplete list.
For external user (client) who need to know parameters of sensor or switch, it’s necessary to know more parameters or configuration of sensor/switch/etc.
i suggest to modify parameter publish_attributes in configuration.yaml with option flat or json or false by default. To keep compatibility, true is equivalent to flat.