Implement stop_extension()

[dlqqq]

Description

Currently AiExtension does not implement the async stop_extension() method. This is causing the server to take much longer than it usually does to stop. When I add this block:

    async def stop_extension(self):
        if 'dask_client_future' in self.settings:
            dask_client: DaskClient = await self.settings['dask_client_future']
            await dask_client.close()

The server stops considerably faster. However, if an exception is raised in this block, then the server refuses to stop without kill -9 <PID>.

We need to determine 1) what else needs to be done (if anything) in this method, and 2) how to safely implement this method and handle exceptions.

[dlqqq]

Also curious as to how Jupyter Server stops an extension that does not implement stop_extension().