feat: CartItem converted to WPInterface type

[kidunot89]

Your checklist for this pull request

Thanks for sending a pull request! Please make sure you click the link above to view the contribution guidelines, then fill out the blanks below.

🚨Please review the guidelines for contributing to this repository.

• Make sure you are making a pull request against the develop branch (left side). Also you should start your branch off our develop.
• Make sure you are requesting to pull request from a topic/feature/bugfix/devops branch (right side). Don't pull request from your master!
• Have you ensured/updated that CLI tests to extend coverage to any new logic. Learn how to modify the tests here.

What does this implement/fix? Explain your changes.

Converts the CartItem to a WPInterface type, migrating the functionality from WooGraphQL Pro for better compatibility between the two plugins.

Does this close any currently open issues?

…

Any relevant logs, error output, GraphiQL screenshots, etc?

(If it’s long, please paste to https://ghostbin.com/ and insert the link here.)

Any other comments?

…

Where has this been tested?

• WooGraphQL Version: …
• WPGraphQL Version: …
• WordPress Version:
• WooCommerce Version: