[PERF]: Use 256 kvikio threads by default in cudf-polars

[Matt711]

kvikio's thread pool defaults to 8 threads (KVIKIO_NTHREADS). We're seeing much better results with 256 kvkio threads in our cloud benchmarks.

SF300, Q1 hot times (S3)

Config	Default	256 threads
meta prefetch off	~1811s	~153s
meta prefetch on	~1641s	~86s

SF300, Q1 hot times (NVMe)

Config	Default	256 threads
meta prefetch off	~29s	~29s
meta prefetch on	~33–38s	~33–38s

*These numbers we're done with #23558

So we'll set the default to 256 via kvikio.defaults.set("num_threads", 256) at engine startup. And make it overridable via CUDF_POLARS__EXECUTOR__KVIKIO_NTHREADS. If unset, we'll fall back to KVIKIO_NTHREADS, then 256.

xref #23551