Add support for Cost of Goods Sold (COGS)

## User story
WooCommerce 9.9 introduced Cost of Goods Sold (COGS) for all products. Since 10.3 it is a stable feature. We should sync this data with GMC to benefit from it (e. g. automated discounts).

### Describe the solution you'd like 
Sync the new COGS field with GMC (`cost_of_goods_sold`)