feat: Add Adyen Payment App

[vijayraghav-io]

What does this PR do?

• Fixes #16860
• Fixes CAL-4427

/claim #16860

https://www.loom.com/share/dd294f65d58c432eb7d74ecca2e0fac0?sid=4a17a78a-ac27-4780-a596-772e8ca1c41f

Mandatory Tasks (DO NOT REMOVE)

• [x] I have self-reviewed the code (A decent size PR without self-review might be rejected).
• [ ] I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. If N/A, write N/A here and check the checkbox.
• [ ] I confirm automated tests are in place that prove my fix is effective or that my feature works.

Refer (few key docs) for setting up Adyen account in live and test, creation of OAuth App - https://docs.adyen.com/development-resources/oauth/ https://docs.adyen.com/development-resources/oauth/integration/#integration-steps https://docs.adyen.com/development-resources/oauth/scopes/ https://docs.adyen.com/development-resources/webhooks/verify-hmac-signatures/?programming_language=js https://docs.adyen.com/development-resources/testing/test-card-numbers/

Few tips, if testing locally - Adyen accepts only public urls (http://localhost will not work) for webhook creation even in test env. So in these files ${WEBAPP_URL} must be replaced with public urls if ngrok is used (temporarily for local testing).

• cal.com/packages/app-store/adyen/api/callback.ts
• cal.com/packages/app-store/adyen/pages/setup/index.tsx