title: Add restriction to WooCommerce coupons by allowed user ID
body: <p>Let's say i'm using this code to create a coupon:</p>
  <pre class="lang-php prettyprint-override"><code>$coupon_code = 'CODEOFF';
  $amount = '10';
  $discount_type = 'percent';
                      
  $coupon = array(
      'post_title' =&gt; $coupon_code,
      'post_status' =&gt; 'publish',
      'post_author' =&gt; 1,
      'post_type'     =&gt; 'shop_coupon'
  );
                      
  $new_coupon_id = wp_insert_post( $coupon );
                      
  update_post_meta( $new_coupon_id, 'discount_type', $discount_type );
  update_post_meta( $new_coupon_id, 'coupon_amount', $amount );
  update_post_meta( $new_coupon_id, 'individual_use', 'yes' );
  update_post_meta( $new_coupon_id, 'product_ids', $some_id );
  update_post_meta( $new_coupon_id, 'usage_limit', '1' );
  update_post_meta( $new_coupon_id, 'expiry_date', $some_date );
  </code></pre>
  <p>Now I want to limit this code for only one user.</p>
  <p>There is an option to use <code>update_post_meta( $new_coupon_id, 'customer_email', '' );</code>, but my users don't have emails.</p>
  <p>How can I <strong>limit this coupon to a user using that user's ID?</strong></p>