[HPOS] Allow admins to filter the Subscriptions List Table when HPOS is enabled.

[mattallan]

Fixes #334

Description

The Subscriptions List Table allows store managers to filter subscriptions by product, customer and payment method, however, these filters weren't working when HPOS is enabled:

On non-HPOS stores, we are hooking onto the request filter and modifying the WP Query to return results based on the filters set in GET params. However this request filter isn't used on the new WC Orders List Table page.

On stores that have HPOS enabled, WooCommerce has provided a new hook that lets us filter the table results by modifying the params sent to wc_get_orders().

This PR fixes being able to filter the Subscriptions List Table by implementing the following changes:

• New function to the woocommerce_shop_subscription_list_table_prepare_items_query_args hook which filters the query args that get passed to wc_get_orders().
• Moved the code that was used to filter the request to filter customers, products and payment methods into separate functions.
• These functions can be repurposed to work for both filtering the List Table for stores that have HPOS enabled and disabled.
• Updated the DB queries called in wcs_get_subscriptions_for_product() to also work on stores that have HPOS enabled.

How to test this PR

1. Purchase multiple different subscription products to help with testing the filtering by product
2. Manually create/add a subscription via the Admin page with a manual payment method
3. Go to WooCommerce > Subscriptions
4. With HPOS enabled:
5. Filter the subscriptions table by product
6. Filter the subscriptions table by customer (also check for customers that have no subscriptions)
7. Filter the subscriptions table by payment method (check "none", "woocommerce payments" and "manual payments")
8. Run through the same tests with HPOS disabled.

Product impact

• [x] Added changelog entry (or does not apply)
• [x] Will this PR affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• [x] Will this PR affect WooCommerce Payments? yes/no/tbc, add issue ref
• [ ] Added deprecated functions, hooks or classes to the spreadsheet

[james-allan]

These changes look good to me and work well.

I've tested filtering the subscription list table on HPOS and CPT environments using various combinations of payment method, customer and product. All searches returned appropriate results.