Provide a hook for clean specification of additional fields, without
relying on risky default dict method parameters.

Use the existing `_group_by_sale` to cleanly add additional
"group by" information.

Partial backport of 740a507d

closes odoo/odoo#105246

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>
Co-authored-by: Victor Feyens <vfe@odoo.com>