Allow to filter export by product.

[sun]

Problem

• The export cannot be filtered to a certain product.

Details

• The submit handler was already foreseeing this in some way, only the export form does not contain a corresponding field for it.
• Luckily we can piggyback on the WooCommerce product filter autocomplete behavior, too. :)

Proposed solution

1. Add the form field to select a product to filter by.
2. Form submission handling was already implemented by someone else in the past.

Notes

• The wcs_get_subscriptions() API only supports a single product ID - but that is at least better than nothing:
  https://github.com/riclain/woocommerce-subscriptions/blob/b0dd2efd869b0639ad54131fba9c13b4ed5a09a2/wcs-functions.php#L414-L417

[sun]

Any update on this? We're using the code in production already.

[sun]

We are still using this successfully.