[IMP] stock: `inventory_quantity_set` own compute

Sets the `inventory_quantity_set` as an editable field (not readonly)
and creates its own compute to avoid to override it each time the
`inventory_quantity` is modify and we set the `inventory_quantity_set`
aside (before this commit, this field new value was always overrided by
the `inventory_diff_quantity` compute).
This is useful when we want to write on the quants from the Barcode App
as we can write on `inventory_quantity` and `inventory_quantity_set` at
the same time.

task-2632818

closes odoo/odoo#76756

Related: odoo/enterprise#20985
Signed-off-by: Rémy Voet <ryv-odoo@users.noreply.github.com>