[Enhancement]: Add a filter for the items to apply coupons array.

[rawdreeg]

Describe the solution you'd like

I am working on an issue where fixed_cart coupon needs to be applied to only a subset of the cart items. The items to apply coupon array is built by get_items_to_apply_coupon method in WC_Discounts class.

I propose a filter in get_items_to_apply_coupon modify the items to apply coupons to.

Describe alternatives you've considered

I initially noticed that $coupon->is_valid_for_product is called to check whether to exclude an item so I attempted using woocommerce_coupon_is_valid_for_product filter, but that's not enough because the exclusion only applies to non-cart coupon.

Additional context

No response