Implement API

[kchodorek]

Summary

Recently, we've been working on a Solidus-based project that uses Solidus via an API. To enable Stripe payments within this setup, we needed to add additional API endpoints and logic for creating payment intents within Stripe. The intent information can later be used in combination with the Stripe Payment Element.

This PR aims to integrate these changes into the upstream solidus_stripe.

It adds the following API endpoints:

• Create Setup Intent
• Create Payment Intent

More details on these endpoints can be found in the README here.

The core logic of this change is encapsulated in service objects, making it easy to create these intents outside of the API as well (e.g., if you prefer a custom GraphQL mutation or need to create a payment intent outside of the regular checkout process for some reason).

Checklist

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• [x] I have written a thorough PR description.
• [x] I have kept my commits small and atomic.
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• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
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[kchodorek]

@jarednorman Thanks for your comment! I adjusted the specs, now instead of stubbing, I'm using stripe-ruby-mock gem, tests are much cleaner now.