feat: Subscriptions with Stripe function

[loks0n]

What does this PR do?

Adds a function to demo integration with Stripe.

• Database collection 'Subscribers' to track user subscriptions
• /checkout endpoint takes users to a stripe checkout page, where users can pay for Premium membership.
• /webhook endpoints to upgrade/downgrade the user based on stripe events.

Test Plan

You can run the function locally by creating the neccessary .env and executing npx appwrite-function-utils dev src/main.js

armang4.appwrite.org link:

Have you read the Contributing Guidelines on issues?

Yes