Add new webhooks for safety alert notifications

Introduce two new webhook events: one for approved safety alerts available at the API project level and another for alerts at the enterprise workspace level. Each webhook sends an event payload containing an alert ID, which can be retrieved using appropriate API keys with `api.safety.alerts.read` or `chatgpt.enterprise.safety_alerts.read` permissions, enabling clients to be notified and fetch relevant safety alerts programmatically.

OpenAI-API-Ref-Source-Revision: 1029816