Using filters to collect cart endpoint data using StoreAPI

[wjrosa]

Changes proposed in this Pull Request:

This is a simple PR that makes use of the new filter introduced in #3780 (wcstripe.express-checkout.cart-add-item) to replace some parts of the code. It is heavily inspired by the WooPayments version.

Testing instructions

• Checkout and build this branch on your test environment (dev/implementing-filters-when-calling-cart-store-api)
• Connect your Stripe account
• Enable express payment methods (Google Pay, Amazon Pay, Link, Apple Pay)
• As a shopper, access the store using a public-facing address
• Confirm you can purchase products using the product, cart, and checkout pages. On both classic/shortcode and blocks version

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• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)