Add DiscountsMeta slot to Checkout sidebar

[opr]

Following on from #4248 this PR will add the same Slot to the Checkout sidebar.

Screenshots

How to test the changes in this Pull Request:

1. Check out feature/add-inputs-for-points-redemption from https://github.com/woocommerce/woocommerce-points-and-rewards/ and ensure you've run npm run build in that project.
2. Go to the file includes/class-wc-points-rewards-extend-store-endpoint.php in Points and Rewards and comment out the following lines (66-72):

self::$extend->register_update_callback(
  array(
    'endpoint'          => CartExtensionsSchema::IDENTIFIER,
    'namespace'         => self::IDENTIFIER,
    'callback_function' => array( 'WC_Points_Rewards_Cart_Checkout', 'rest_apply_discount' ),
  )
);

3. Ensure you have added points to your user account ( WooCommerce -> Points and Rewards) and that you're logged in
4. Add items to your cart and visit the Checkout block. See that the Points and Rewards input is present. Note that it won't function on this branch. This PR is just to get the display working.

Changelog

Add Slot in the Discounts section of the Checkout sidebar to allow third party extensions to render their own components there.

[github-actions]

Size Change: +441 B (0%)

Total Size: 988 kB

Filename	Size	Change
build/active-filters-frontend.js	7.98 kB	+1 B (0%)
build/active-filters.js	7.53 kB	+1 B (0%)
build/all-products-frontend.js	34.7 kB	+1 B (0%)
build/all-products.js	36.5 kB	-1 B (0%)
build/all-reviews.js	9.29 kB	+134 B (+1%)
build/atomic-block-components/image-frontend.js	1.66 kB	-1 B (0%)
build/atomic-block-components/sale-badge-frontend.js	469 B	-1 B (0%)
build/attribute-filter-frontend.js	17.7 kB	+2 B (0%)
build/attribute-filter.js	11.4 kB	+1 B (0%)
build/cart-frontend.js	77.9 kB	+3 B (0%)
build/checkout-frontend.js	81.9 kB	+22 B (0%)
build/checkout.js	46.6 kB	+24 B (0%)
build/featured-category.js	7.23 kB	-1 B (0%)
build/featured-product.js	9.4 kB	-5 B (0%)
build/price-filter-frontend.js	14.3 kB	+2 B (0%)
build/price-filter.js	9.3 kB	-1 B (0%)
build/product-best-sellers.js	6.12 kB	+1 B (0%)
build/product-categories.js	3.38 kB	+140 B (+4%)
build/product-category.js	6.99 kB	+1 B (0%)
build/product-on-sale.js	6.62 kB	+1 B (0%)
build/product-search.js	2.68 kB	+118 B (+5%)	🔍
build/product-tag.js	6.11 kB	+2 B (0%)
build/products-by-attribute.js	7.22 kB	+2 B (0%)
build/reviews-by-product.js	12.7 kB	-2 B (0%)
build/reviews-frontend.js	8.94 kB	+1 B (0%)
build/single-product-frontend.js	38.1 kB	+2 B (0%)
build/single-product.js	9.65 kB	-1 B (0%)
build/vendors.js	242 kB	-5 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/atomic-block-components/add-to-cart--atomic-block-components/button--atomic-block-components/image---a7e2bb9b.js	2.39 kB	0 B
build/atomic-block-components/add-to-cart--atomic-block-components/button.js	1.95 kB	0 B
build/atomic-block-components/add-to-cart--atomic-block-components/image--atomic-block-components/title.js	334 B	0 B
build/atomic-block-components/add-to-cart-frontend.js	8.74 kB	0 B
build/atomic-block-components/add-to-cart.js	7.8 kB	0 B
build/atomic-block-components/button-frontend.js	1.73 kB	0 B
build/atomic-block-components/button.js	844 B	0 B
build/atomic-block-components/category-list-frontend.js	469 B	0 B
build/atomic-block-components/category-list.js	478 B	0 B
build/atomic-block-components/image.js	1.3 kB	0 B
build/atomic-block-components/price-frontend.js	1.98 kB	0 B
build/atomic-block-components/price.js	2.01 kB	0 B
build/atomic-block-components/rating-frontend.js	521 B	0 B
build/atomic-block-components/rating.js	525 B	0 B
build/atomic-block-components/sale-badge.js	474 B	0 B
build/atomic-block-components/sku-frontend.js	389 B	0 B
build/atomic-block-components/sku.js	393 B	0 B
build/atomic-block-components/stock-indicator-frontend.js	569 B	0 B
build/atomic-block-components/stock-indicator.js	572 B	0 B
build/atomic-block-components/summary-frontend.js	906 B	0 B
build/atomic-block-components/summary.js	910 B	0 B
build/atomic-block-components/tag-list-frontend.js	464 B	0 B
build/atomic-block-components/tag-list.js	472 B	0 B
build/atomic-block-components/title-frontend.js	1.41 kB	0 B
build/atomic-block-components/title.js	1.26 kB	0 B
build/blocks-checkout-editor.js	10.6 kB	0 B
build/blocks-checkout.js	19.8 kB	0 B
build/blocks.js	3.51 kB	0 B
build/cart.js	44.3 kB	0 B
build/editor-rtl.css	14.9 kB	0 B
build/editor.css	14.9 kB	0 B
build/handpicked-products.js	5.89 kB	0 B
build/price-format.js	1.37 kB	0 B
build/product-new.js	6.28 kB	0 B
build/product-top-rated.js	6.26 kB	0 B
build/reviews-by-category.js	11.2 kB	0 B
build/style-rtl.css	18.9 kB	0 B
build/style.css	18.9 kB	0 B
build/vendors--atomic-block-components/price-frontend.js	6.54 kB	0 B
build/vendors-style-rtl.css	1.05 kB	0 B
build/vendors-style.css	1.05 kB	0 B
build/wc-blocks-data.js	10.8 kB	0 B
build/wc-blocks-google-analytics.js	1.99 kB	0 B
build/wc-blocks-middleware.js	1.48 kB	0 B
build/wc-blocks-registry.js	2.73 kB	0 B
build/wc-payment-method-bacs.js	812 B	0 B
build/wc-payment-method-cheque.js	807 B	0 B
build/wc-payment-method-cod.js	903 B	0 B
build/wc-payment-method-paypal.js	844 B	0 B
build/wc-payment-method-stripe.js	12.4 kB	0 B
build/wc-settings.js	2.93 kB	0 B
build/wc-shared-context.js	1.54 kB	0 B
build/wc-shared-hocs.js	1.73 kB	0 B

_{compressed-size-action}

[frontdevde]

Tested as per testing instructions and can confirm that the points show up in the Checkout sidebar as expected:

LGTM 👍