Add cart quantity controls and expose preload cache invalidate function

What changes are you making?

Exposes an invalidate function that clients can call to invalidate the preload cache.

Adds quantity controls to the cart page in the MobileBuyIntegration sample app. Calls invalidate when they're used.

https://github.com/user-attachments/assets/1d679d96-d462-4779-bf0c-3dfcf473ad85

How to test

Start it up and test it out

Before you merge

[!IMPORTANT]

[x] I've added tests to support my implementation

[x] I have read and agree with the Contribution Guidelines.

[x] I have read and agree with the Code of Conduct.

[x] I've updated the README.

Checklist for releasing a new version

- [ ] I have bumped the version number in the [`build.gradle` file](https://github.com/Shopify/checkout-kit-android/blob/main/lib/build.gradle#L17) - [ ] I have added a [Changelog](https://github.com/shopify/checkout-sheet-kit-android/blob/main/CHANGELOG.md) entry. - [ ] I have updated the versions in the [README.md](https://github.com/shopify/checkout-sheet-kit-android/blob/main/README.md) for both Gradle and Maven.

[!TIP] See the Contributing documentation for instructions on how to publish a new version of the library.

Shopify / checkout-sheet-kit-android