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
closesodoo/odoo#116498
Related: odoo/upgrade#4473
Related: odoo/enterprise#38692
Signed-off-by: Lucas Lefèvre (lul) <lul@odoo.com>