message: Add changelog entry for the new JSON types
diff: @@ -5,6 +5,11 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  ## [Unreleased]
  +### Added
  +
  +- The new classes `json_value`, `json_array` and `json_object` allow working
  + with JSON inputs directly. Actors can also pass around JSON values safely.
  +
  ### Fixed
  - Passing a response promise to a run-delayed continuation could result in a
  