adding stripe modular example

[tylernix]

Description

This example models a few of Stripe's core functionality, but not everything. It introduces another modular models example splitting the model into 3 modules:

• accounts.fga - core Stripe account functionality
• payments.fga - (yet to be implemented)
• issuing.fga - credit card issuing and transactions

It also introduces modeling concepts such as "self-defining" relationships, "dual-write" relationships, and "intermediate" relationships. It also uses exclusions and conditions.

It includes tests for two scenarios:

1. SCENARIO 1: An account manages 2 cards, assigned to users Raghd and Jon. Raghd's card is active and makes two purchases. Jon's card is inactive and makes one purchase
2. SCENARIO 2: A Stripe account (in test mode) manages one active card that tries to make a purchase.

• [ ] Working on writing a README.md for the sample store.

Review Checklist

• [ ] I have clicked on "allow edits by maintainers".
• [ ] I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• [ ] The correct base branch is being used, if not main
• [ ] I have added tests to validate that the change in functionality is working as expected