This feature branch adds a new way to add products to SOs/POs: the grid (or matrix) entry.

When selecting a product template compatible with this mode, a dialog will open with a matrix of
available variants and a number input for each of those. Once all quantities are input, closing the
dialog will generate the lines for all the selected variants.

When more than 2 attributes are present, the matrix is organized by using the first attribute values
as X-coordinates and will then combine all other attribute combinations as Y-coordinates. This
allows to have potentially huge matrices, although this could be difficult to visually parse by users.

If the feature is enabled (the feature is installed independently in sale and/or purchase), this will be
the default entry mode for products in purchase orders when multiple variants are available - it's an
all or nothing situation. In sales orders, the product template can be configured to switch from the
product configurator mode (to select a single variant when adding the product) to the grid mode (to
select multiple variants at once).

The grid can be re-opened from an existing line (or by creating a new line with the same product)
and will repopulate with the current quantities of the order, allowing quick massive edition of the
complete set of lines for that product.

Task 1871560

closes odoo/odoo#34746

Signed-off-by: Damien Bouvy (dbo) <dbo@odoo.com>