No designated place for traceback in message item

Steps to reproduce:

• set send_message_trace to true,
• report a message item Rollbar::log(Level::INFO, 'Message from app');

Expected result
The trace back should be visible in the Rollbar item details page just as when reporting an exception.

Result
The trace back is not visible in the Rollbar item details page. It's under body.message.backtrace in unformatted and hard to read in occurrence details.

Impact across SDKs
This is a problem that will appear across other SDKs as well as currently we just don't support backtrace for message items. Support for that needs to be implemented.