feat: support async_approval_notification_channels for CIBA on clients

[kushalshit27]

🔧 Changes

Adds support for the async_approval_notification_channels property to the auth0_client resource for managing CIBA (Client-Initiated Backchannel Authentication) notification channels.

This property allows configuration of which notification channels are enabled for CIBA requests initiated by a client. The order of channels is significant as it determines the evaluation order.

Example usage:

resource "auth0_client" "my_client" {
  name     = "My Application"
  app_type = "non_interactive"

  # CIBA notification channels (order matters)
  async_approval_notification_channels = [
    "guardian-push",
    "email"
  ]
}

📚 References

🔬 Testing

• Unit tests pass
• Acceptance test added: TestAccClientAsyncApprovalNotificationChannels
  • Tests creation with single channel
  • Tests update to multiple channels
  • Tests order preservation

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)