feat: add optional fetcher key to CartForm

[rmiller61]

WHY are these changes introduced?

Improve developers' experience by giving them access to a CartForm's fetcher outside the component where the fetcher is created.

For example, if I have a CartFrom that opens a drawer after a product has been added to the cart, I would like to display a pending UI while the form route is being called. To do this, I need to know the value of a fetcher.state that is scoped outside of the modal.

By passing a key to the CartForm's useFetcher hook, I can access that fetcher anywhere in my app:

// app/components/ProductForm.tsx

<CartForm fetcherKey="MY_FETCHER_KEY" route="/cart" action={CartForm.ACTIONS.LinesAdd}>

     // Render the inputs

     <button type="submit" onClick={ () => openDrawer() }>Add to Cart</button>

</CartForm>

// app/components/CartDrawer.tsx

const fetcher = useFetcher({ key: "MY_FETCHER_KEY" });

if (fetcher.state !== 'idle') {
     // Pending UI
}

WHAT is this pull request doing?

Adds an optional fetcherKey prop to packages/hydrogen/src/cart/CartForm.tsx and passes it to the component's useFetcher hook.

HOW to test your changes?

• Build @shopify/hydrogen and run a demo app.
• Since CarForm is an abstraction of an existing API (useFetcher()) and this feature builds on top of that API, it shouldn't affect test coverage

Post-merge steps

Checklist

• I've read the Contributing Guidelines
• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added a changeset if this PR contains user-facing or noteworthy changes
• I've added tests to cover my changes
• I've added or updated the documentation

[rmiller61]

I have signed the CLA!

[wizardlyhel]

Thanks for the contributions