Add webhook sender with throttling and retry
WebhookEventType enum for filtering (start, output, logs, completed).
WebhookConfig with throttle interval, retry settings, status codes.
TraceContext for W3C distributed tracing headers.
WebhookSender implements:
- send(): fire-and-forget for non-terminal events with throttling
- send_terminal(): async with exponential backoff retries
- send_terminal_sync(): blocking version for Drop contexts (uses ureq)
Bearer auth via WEBHOOK_AUTH_TOKEN env var.
