dev: implement the set_checkpoint function for Generic Entry

[JordyRo1]

The function set_checkpoint realizes a match according to the input parameter. It then stores a pack form of Checkpoint. For now, only Spots and Futures are handled. We need to extend it for Generic Entry, but for now, we do not have a usage for a specific generic type. Thus the parameters saved in Checkpoint (value, timestamp, num_sources_aggregated, aggregation_mode), might not be relevant to save for a generic entry. Once we have a solid usage for Generic Entry, we might be able to determine the parameters to save, and write a general implementation.