[ADD] pos_uom: Add secondary unit of measure in POS

[pamd-odoo]

Introduced the ability to use a secondary unit of measure in Point of Sale.

• A new button allows entering quantities in the secondary UoM.
• Users can input the quantity in the popup, and the system converts it into the primary UoM automatically.
• The orderline shows the converted quantity in the main unit, making sales more flexible for products sold in different measures.

[robodoo]