[FIX] fields: add group_operator to be availble in get_description.

Web client use fields_get (which one call get_description) to know if
a group operator exists. But until now, group_operator are never returned.

Without it, the web client cannot display the sub-total except for sum
(the fallback in the web client).

This commit closes #13713

Todo: do the same on Class Monetary in next branch