[IMP] base: support `group_expand` for custom fields

Allow a naive `group_expand` for manual fields where having the
attribute set to True causes the ORM to include all records from the
relation model of the m2o field in the read_group.

This is particularly useful for custom m2o fields which represent
stages - a grouped list view or a kanban view should include all
possible stage, not only the currently used values.

Co-Authored-By: Raphaël Collet <rco@odoo.com>