feat: Add Stripe API

[jonas-martinez]

About this PR

Linked to lenra-io/lenra_cli#320

In this PR, I have added API definitions to properly call the new endpoints on the server for Stripe subscriptions handling.

I added these API definitions:

• POST /stripe/customers -- To Create or Get the Customer for the current user.
• GET /stripe/subscriptions -- To get the subscription or nothing for the current application of the user.
• POST /stripe/checkout -- To proceed to the Stripe payment page for the selected subscription.
• GET /stripe/customer_portal -- To proceed to the Stripe Customer portal which makes it possible to manage your subscriptions across all of your applications.

Checklist

• I didn't over-scope my PR
• My PR title matches the commit convention
• I did not include breaking changes
• I made my own code-review before requesting one

I included unit tests that cover my changes

• 👍 yes
• 🙅 no, because they aren't needed
• 🙋 no, because I need help

I added/updated the documentation about my changes

• 📜 README.md
• 📕 docs/*.md
• 📓 docs.lenra.io
• 🙅 no documentation needed