This commit refactors the current model structure of spreadsheet-related module. Data fields are integrated into an abstract model `spreadsheet.mixin` and all sub-models are inherited from it. This way we can group all decode/encode logics into one place. task 3222572 closes odoo/odoo#116498 Related: odoo/upgrade#4473 Related: odoo/enterprise#38692 Signed-off-by: Lucas Lefèvre (lul) <lul@odoo.com>