Add example on how to iterate on child to FirebaseDemo_ESP8266

[proppy]

Showing how to use FirebaseArduino.getVariant and JsonObject iterators:
https://arduinojson.org/v5/api/jsonobject/begin_end/

[am-canes]

@proppy could you give me an example?

I am trying to read a node and iterate on child like this:

FirebaseObject child = Firebase.get("config");
JsonObject& obj = child.getJsonVariant();

for (JsonObject::iterator it=obj.begin(); it!=obj.end(); ++it)
{
Serial.println(it->key);
}
But it doesn't return any value.

[cwhokok]

am-canes's source works good. thank you.