Add WooCommerce Order ID to Sync Options

order_id is already supported but it’s not actually the order ID, it’s the order number (which might also need correcting in the field labels on the Contact Fields tab). Many stores use consecutive order numbers instead of the order (or WP Post) IDs for customer-facing communication which means the WooCommerce order ID isn’t always the same as the order number. If we wanted to use the CRM’s emails for admin notifications we would require the internal order ID rather than the order number to be able to build a link.