Allow use of obstore as a Zarr store

[tomwhite]

Fixes #715

All tests pass with

CUBED_SPEC__STORAGE_OPTIONS__USE_OBSTORE=True pytest

But note this doesn't use obstore for all tests, since many have hardcoded Spec objects.

[tomwhite]

I ran the benchmarks on a local machine with an SSD, using the following local_processes_obstore.yaml:

spec:
  work_dir: "$PWD/temp/"
  allowed_mem: "2GB"
  executor_name: "processes"
  storage_options:
    use_obstore: True

export CUBED_CONFIG=tests/configs/local_processes_obstore.yaml
pytest -vs --benchmark

The time was essentially the same as #492 (comment) (and for plain Zarr Python v3 - which again had similar numbers)

┌─────────────────────────────────────────────────┬────────────────────────────┬────────────────────┐
│                      name                       │           start            │      duration      │
│                     varchar                     │         timestamp          │       double       │
├─────────────────────────────────────────────────┼────────────────────────────┼────────────────────┤
│ test_quadratic_means_xarray[50-new-optimizer]   │ 2025-06-27 11:14:36.641891 │ 2.9726290702819824 │
│ test_quadratic_means_xarray[500-new-optimizer]  │ 2025-06-27 11:15:26.2486   │   30.7053439617157 │
│ test_quadratic_means_xarray[5000-new-optimizer] │ 2025-06-27 11:17:21.658876 │  291.1440191268921 │
├─────────────────────────────────────────────────┼────────────────────────────┼────────────────────┤

It will be interesting to see if obstore does better on high-latency stores like S3.

[tomwhite]

I just re-ran this with the latest code (obstore 0.8.1), and using the obstore Zarr store is now 12% faster than the default local store on this benchmark (252.9s vs 287.7s).

┌─────────────────────────────────────────────────┬────────────────────────────┬────────────────────┐
│                      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 │
└─────────────────────────────────────────────────┴────────────────────────────┴────────────────────┘

[tomwhite]

See #731 (comment) for comparison with zarrs-python.