Update a callback endpoint

Replaces an endpoint's destination after applying the complete URL and DNS policy, and renames it when a name is given. Optional headers patch endpoint defaults with set, keep, or remove; omitted headers preserve them. A secret value can be retained, replaced (including an empty string), or removed, but cannot be revealed. Header and destination edits apply to newly admitted attempts, including retries; recovery of the same admitted attempt keeps its snapshot. Disabled endpoints cannot be updated. Send If-Match with the version the endpoint was read at to be refused rather than overwrite a destination somebody else changed in the meantime.