🚀 Feature: OAuth2 Support for OpenAI (ChatGPT)

Problem
Today, OpenAI integration in n8n relies on API keys, which:

Don’t support user-level authentication
Don’t integrate with accounts that use subscriptions like ChatGPT Plus

:light_bulb: Proposed Solution
Add OAuth 2.0 authentication for OpenAI credentials.

Flow:
Click “Connect with OpenAI”
Redirect to OpenAI login/consent
Receive authorization code
Exchange for access/refresh tokens
Store securely and use in nodes

:fire: Why it matters
Secure authentication (no API keys)
Better UX
Enables usage tied to user accounts (e.g., ChatGPT Plus subscribers)

:brain: Use cases
Users who already pay for ChatGPT Plus and want native integration