Add frontend plugin to allow for a custom React component to be rendered under each message

[dlqqq]

Problem

Some developers configuring Jupyter AI with their own custom extension would like to be able to render a custom React component under each message in the UI. This allows for a wide range of capabilities, such as rendering a feedback button under each AI message.

Proposed Solution

• Export a new token & type, e.g. MessageAddition. The type expected by Jupyter AI should be that of a React component which takes a single message: AiService.ChatMessage prop.
• Optionally request an implementation by listing it within the optional field of the root plugin object in packages/jupyter-ai/src/index.ts.
• If this component is provided, then render this under each message in the chat.

[michaelchia]

when this is done, would it be possible to provide an example in the docs or elsewhere how to implement feedback buttons with this feature?