[REF] point_of_sale: load_server_data improved

First approach to make PosModel load_server_data inheritable. I am adding a new module that stores all the fields that are fetched on load_server_data. This is because sometimes we need to load more fields for some object. Like res.partner or company. If we need more fields we have to fetch them again and it's not the optimal way. With new module the developer can attach his fields to be fetched on the model so he is going to be able to get more data without fetching that data again.