homeassistant service discovery: derive node_id when using nested topics (#3088)

* derive correct node_id for homeassistant service discovery in nested topics (fixes #1792)

* explicit use of std::string

* move nodeId creation to separate function
add unit-tests

* add documentation about node_id generation for Home Assistant MQTT Service Discovery

code/components/jomjol_mqtt/server_mqtt.h

@@ -22,6 +22,7 @@ std::string getTimeUnit(void);
 void GotConnected(std::string maintopic, bool SetRetainFlag);
 esp_err_t sendDiscovery_and_static_Topics(void);
 
+std::string createNodeId(std::string &topic);
 
 #endif //SERVERMQTT_H
 #endif //ENABLE_MQTT