[ADD] models: add _read_group method to avoid sorting on the names of many2one

 The public read_group method is used by back-end code as well as RPC calls to receive grouped data.
 Most of the time, if the data isn't shown to the user, it doesn't need to be sorted on the names of
 the many2one, and can just be sorted on their ID instead, avoiding extra joins in the resulting queries

 This is a first commit that will reduce the features of _read_group to the strict minimum necessary
 to obtain correct grouped data that is never shown to a user

Task-id: 2479334
Part-of: odoo/odoo#84908