[WIP] Encode the message only once

[pulzarraider]

The Pub/Sub message generated from this bundle is currently double JSON-encoded. This adds unnecessary complexity for other applications when processing the messages and introduces avoidable complications.

It can be simplified to just one JSON-encoding. Since this represents a significant backward compatibility (BC) break, the change must be included in the next major release.

TODO

• For the 3.x branch, add a compatibility option that allows encoding messages using the new format, while still supporting decoding of both the old and new formats. This ensures no messages are lost during the upgrade process: [WIP] Encoding strategy for 3.x #64
• Decide what to do with AttributesStamp. It may not be needed as the message headers are attributes.

Benefits

• Improves performance (message is json encoded/decoded only once).
• Enables support for sending binary data with custom serializer, mentioned in Option to send/receive binary data #17.