This is a backwards compatible message format. It adds a version field to the message, and fallbacks to the existing envelope as a default. This will allow to deploy a worker that supports the current in-flight messages and also a new format. The goal is to eventually phase out the legacy format (the one without a version field), and to add a version that doesn't need the S3 keys nor the hydratable attributes (the former is not needed here, but can't yet be removed because of backwards compatibility, the later is an artifact of how the messages are constructed, and it is not important for the worker, it also results in compilation warnings because of the macro use)