[Example]: RSC/SSR Cart with session

[juanpprieto]

Description

A hydrogen example demonstrating a 💯 .server-driven basic Cart workflow.

• Provides a set of cart utilities (getCart, createCart...) as well as a GraphQL cart client
• Provides a /api/cart/[action] that handle POST requests from the different get, create, add and remove form actions.
• Persists cartId and cartCount to the session (cookie), so they can be used used during the .server lifecycle
• Uses form actions for all interactions with the API. e.g — AddToCart, remove..
• Provides suspendedFn a utility to enable <Suspend /> friendly async API calls from .server components

Additional context

Oxygen Demo

---

Before submitting the PR, please make sure you do the following:

• Read the Contributing Guidelines
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123)
• Update docs in this repository according to your change
• Run yarn changeset add if this PR cause a version bump based on Keep a Changelog and adheres to Semantic Versioning