Cancel execution / stop streaming responses

[jtpio]

Problem

Users may want to be able to stop the model from generating content, for example if the answer is not satisfying or this there was an error in the prompt.

Proposed Solution

Implement the approach mentioned in langchain docs to cancel execution: https://js.langchain.com/docs/how_to/cancel_execution/

Additional context

For the UI element, this would likely require jupyterlab/jupyter-chat#170, unless we can add that button here in an extension directly.