Feature: update Whatsapp embedded signup to allow connect an existing WhatsApp Business app account to Cloud API

Added

WhatsApp Embedded Signup Enhancement: Updated the embedded signup flow to support connecting an existing WhatsApp Business App account to the Cloud API.
- Simplified onboarding for users migrating from WhatsApp Business App to Cloud API.
- Users can now link their current WhatsApp numbers and business profiles without creating a new WABA.

Improved

Enhanced user experience by streamlining the integration process for existing WhatsApp Business accounts.
Added clear instructions and validation checks during the account linking process.

🔗 Seamless upgrade path from WhatsApp Business App to Cloud API is now available!