add FEED_FACTOR to json config

[Friedjof]

Explanation

So that only a new configuration has to be entered, it should also be possible to set the FEED_FACTOR from the main.cpp in the config.json. The FEED_FACTOR is responsible for the calibration of the feed quantity in grams.

TODO

To do this, the ConfigManager (ConfigManager::save_config() and ConfigManager::load_config()) and the associated feed_config_t type must be adapted.