Issue: MessageExtras support for outbound extras and compliance with TM2i

Issue: MessageExtras support for outbound extras and compliance with TM2i: a task in LegoFlow-SWE (Harbor dataset). Since the Message.extras field was changed from JsonObject to MessageExtras in version 1.2.0, the MessageExtras class is incomplete and does not meet the requirements of the TM2i…

The task

Since the `Message.extras` field was changed from `JsonObject` to `MessageExtras` in version 1.2.0, the `MessageExtras` class is incomplete and does not meet the requirements of the TM2i specification. Users cannot create instances with arbitrary JSON data, serialize them, or reliably compare them.

Part of Lego-X/LegoFlow-SWE.