Custom Fields for New Product Editor

[j111q]

Description

A custom fields section, similar in functionality to the "Custom Fields" metabox in the classic experience, will be implemented:

Design
Figma: vhp1LFki2xHqjck6UegZJT-fi-86_55852
P2 post: peCask-DS-p2

Implementation Notes

• post_custom_meta_box() - This is how the classic editor "Custom Fields" metabox is implemented. A great reference.
• We will need to likely create a REST endpoint to get the possible meta field keys to display in the dropdown, as I don't think this is exposed in any way in the core WP REST API.

Acceptance Criteria

• Use custom fields toggle appears under Attributes
• When toggled on, the Custom fields section appears
• User is able to click on the help icon to read helper text, next to the Custom fields header
• Any non-protected meta fields should be shown
• When there are no Custom fields, the empty state should be the same as the one used in Attributes
• When adding or editing custom fields, the modal and interactions should be the same as the one used in Attributes
• If the Use custom fields toggle is on > Custom fields are added > Product is saved > Then Use custom fields is toggled off > Product is saved > Then toggle is switched on again > The previously added fields should re-appear. This would match how it works currently in the legacy editor.