[FIX] stock: allow creating a scrap with more than 2 decimals in qty

scrap_qty field was missing the digits attribute, and therefore the user
could not create a scrap with a quantity with more than two decimal places.
With this commit, scrap_qty will follow the 'Product Unit of Measure'
precision setting, just like any other quantity fields.

closes odoo/odoo#77652

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>