No way to make `execute` clean up client when `km` is passed

When passing `km=...` to `nbclient.execute`, it won’t clean up its kernel client.

`nbclient.NotebookClient.execute` passes its kwargs to `(async_)setup_kernel`, where `cleanup_kc` defaults to the value of `self.owns_km`.

But `nbclient.execute` has no way to pass `cleanup_kc` if you want `km` to spawn new kernels every time.

Start by tracing nbclient.NotebookClient.execute and nbclient.execute into (async_)setup_kernel, focusing on how cleanup_kc is selected when km is supplied. Confirm the intended call path and add a way for callers passing km to request cleanup, then verify that the kernel client is cleaned up after execution.

Written by the indexing model from the issue text.