Changes proposed in this Pull Request:

This pull request adds the ability to edit the Cost of Goods Sold values for products in the product editor of the classic admin UI. It also adds a (yet incomplete) "Cost" column to the products list table in the admin area.

Simple and variable product editors

The "General" tab of the product editor gets a new cost column that gets tooltip'ed differently depending on whether the product is variable or not:

This field will show a value of zero for products that don't have an explicit value set. There's nothing in the database for these, and get_cogs_value for the corresponding product object will return null; but as per #54458 this is equivalent to zero, and an explicit zero is clearer than an empty value.

Variation editor

Variations get their own cost field. For variations that don't have an explicit cost value set, a placeholder indicating the default value configured for the parent is displayed:

Quick and bulk edit

The cost can be edited using the standard quick and bulk editors too. For variable products the value that gets modified is the default value that gets applied to variations:

Bulk actions for variations

One new bulk action is added for variations to remove the custom cost value of all variations (thus effectively reverting to the default value). Confirmation is requested before proceeding:

💡 An additional added improvement is that now the action selector dropdown will go back to the default "Bulk actions" text after the action has run (or the prompt/confirmation has been cancelled). This is useful when having to retry an action after having supplied invalid data.

Cost column

A new "Cost" column is added to the products list table:

Two new filters are added to customize how this column is rendered, similar to the existing filter for prices: woocommerce_get_cogs_html and woocommerce_empty_cogs_html

For now this column is not sortable, as this would require the addition of a new column in the wc_product_meta_lookup database table. This will be implemented in a separate pull request.

Also, for variable products this column only shows the default cost value for variations. Additional information (like the range of costs defined for existing variations, similarly to how the price column shows a range of variation prices) might be added in an additional pull request too.

How to test the changes in this Pull Request:

1. Enable the Cost of Goods Sold feature if you haven't done that yet (WooCommerce - Settings - Advanced - Features, check the box for "Cost of Goods Sold" and save).

2. Create a few products, simple and variable with some variations.

3. Go to the products table ("Products" in the left menu). Open the regular editor for some of the products, simple and variable. The cost value that you see in the "General" tab should be zero. Verify that you can modify the value and save it, and when you reload the editor the value is still there.

4. Try the quick editor ("Quick edit" link in the product rows in the products table). Verify that the cost shown in the "Cost" field is initially 0, and that you can change it and save, and the value gets persisted too.

5. Try the bulk editor (mark the checkbox for a few products in the products table, select "Edit" in the "Bulk actions" dropdown, then click "Apply"). Verify that the cost is modified for the selected products only if the cost field is set to "Change to:", and that an empty value sets a cost of zero.

6. Open the editor for a variation ("Variations" tab when editing a variable product). Verify that the "Cost" field shows a placeholder with the cost value set for the parent product in the "General" tab (or zero, if none has been set). Verify that you can set a value and save it, and that you can then delete the value and save it too (note that an empty value is not equivalent to zero in this case).

7. Try the "Remove custom costs" bulk action for variations (edit a variable object, go to the "Variations" tab, click the "Bulk actions" dropdown). Try accepting and cancelling the confirmation prompt (it should remove the custom cost of all the variations in the first case, and no costs should be changed in the second case).

8. Verify that the "Cost" column in the products table shows the proper values for each product (zero is indicated with a hyphen).

9. Try the cost column HTML filters with the following snippet:

add_filter('woocommerce_empty_cogs_html', function($html, $product)  {
    return "<p>No cost for " . $product->get_id() . "</p>";
}, 2, 10);

add_filter('woocommerce_get_cogs_html', function($html, $value, $product)  {
    return
        0.0 === $value ?
        $html :
        '<p>Cost for ' . $product->get_id() . ': ' . $value . '</p>';
}, 3, 10);

10. Finally, disable the Cost of Goods Sold feature and verify that none of the added UI elements that you just tested is visible anymore, but you can still edit the rest of the product fields (using regular editor, quick edit, bulk edit, variation editor and variation bulk action).

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