Set default executor in `run_streaming_pipeline`

[TomAugspurger]

As noted by @wence- in rapidsai/cudf#21382 (comment), cudf-polars' use of asyncio.to_thread ends up offloading blocking function calls to a thread from a new ThreadPoolExecutor created by asyncio.

We'd (maybe?) prefer that those functions run on the py_executor passed into run_streaming_pipeline. I haven't thought through all the consequences of this, but it seems reasonable.

Because rapidsmpf creates and controls the event loop, we should be able to get a running loop in the runner closure called by asynico.run around

rapidsmpf/python/rapidsmpf/rapidsmpf/streaming/core/node.pyx

Line 254 in 42eca8b

async def runner():

, and call loop.set_default_executor. Then calls to asyncio.to_thread will run on py_executor.