Add a webhook notification that triggers when a live manifest is ready for a specific encoding. A maximum number of 5 webhooks per Encoding is allowed. For a running live encoding (V2 manifest generator, encoder version 2.276.0 or above) it is propagated asynchronously (eventually consistent, typically sub-second) and does not require a restart; if the manifest is already ready, it fires immediately and exactly once. Org-level webhooks apply on the next start.