Replace topic on onSubscribed with onTopicStatus Observer callback

[paclema]

Right now the callback method MQTTClientCallback::onSubscribed() is triggered when

• MQTT_EVENT_SUBSCRIBED for esp32, so whenever a topic was successfully subscribed.
• After mqttClient.subscribe(t.topic.c_str(), t.qos) for esp8266 right after mqtt connect. No matter if the subscription was performed or not, we pass the topic data status struct where this status is also included

However, we could provide topic subscription changes instead of notifying only on those mentioned cases, we could notify whenever we receive an update or status change for a specific topic, so we could inform every observer for each new topic status change.

[paclema]

It has been added MQTTClientCallback::onTopicUpdate callback method for that in this commit: https://github.com/paclema/MQTTClient/commit/39808134088b8448be8526f3e78f0a28c1a34b39 and also updated for esp8266 right after mqtt connect on: https://github.com/paclema/MQTTClient/commit/41e4d29563ac3c62e239558ae27d4f9d2e8bf7e1