Use zarrs-python for intermediate storage

[tomwhite]

We should try the Rust-based zarrs-python for storing intermediate data to see if it speeds up Cubed. It is installed as a Zarr Python 3 codec pipeline, so should be easy to test.

Thanks to @alxmrs for the suggestion!

[tomwhite]

Using the code in #789, I ran the benchmarks on a local machine with an SSD, using the processes executor:

export CUBED_CONFIG=tests/configs/local_processes.yaml
CUBED_STORAGE_NAME=zarrs-python pytest -vs --benchmark

The results were as follows. The first run is the default local Zarr store (287.7s), the second is the obstore Zarr store (252.9s), and the third is zarrs-python (240.8s).

┌─────────────────────────────────────────────────┬────────────────────────────┬────────────────────┐
│                      name                       │           start            │      duration      │
│                     varchar                     │         timestamp          │       double       │
├─────────────────────────────────────────────────┼────────────────────────────┼────────────────────┤
│ test_quadratic_means_xarray[5000-new-optimizer] │ 2025-09-03 10:44:27.731067 │  287.7307958602905 │
│ test_quadratic_means_xarray[5000-new-optimizer] │ 2025-09-03 10:52:01.361027 │ 252.89893579483032 │
│ test_quadratic_means_xarray[5000-new-optimizer] │ 2025-09-03 11:09:21.771515 │ 240.84668517112732 │
└─────────────────────────────────────────────────┴────────────────────────────┴────────────────────┘

So obstore is 12% faster and zarrs-python is 16% faster than the default local store on this benchmark.