Inception session: product chain data model

@Rachel @enricostn please check it is conform to what we said. Also I created an Epic https://github.com/openfoodfoundation/openfoodnetwork/issues/2228 to start working on the first step but not quite sure yet how to handle this, I asked our awesome @danielle some guidance.
@Kirsten @oeoeaio @maikel @lin_d_hop please have a look and tell if the path we propose (prototype to test that data model, then if all good we break into stories) fits you.