Include next payment value in the fields available to sync for WooCommerce subscriptions.

We currently have fields like “Next Payment Date” and “Subscription Product Name”, but it would also be useful to have “Next Payment Value” (aka, the $ value the user will be rebilled in the next payment.) Especially for cases in which the customer purchased using a recurring product discount coupon. We can use that for rebill reminder emails and to try to recover subscriptions in the “pending-cancel” status.