Handle update_display_data message

[ellisonbg]

The Jupyter kernel message spec allows existing updates to be updated through the combination of two different things:

• The display_data (and maybe execute_result) output can have a transient section with a display_id.
• The update_display_data message can target an existing display_id and provide updated output.

We need to implement this logic in our server extension.

https://jupyter-client.readthedocs.io/en/latest/messaging.html#update-display-data

[3coins]

@ellisonbg
Does this mean, the server outputs need to update one or more saved outputs on the disk? Is there a specific cell output (or code) that can help us re-create and test these kind of updates?