obstore implementations for .getsize and .getsize_prefix

[slowjazz]

closes #3218

• implement .getsize and .getsize_prefix for ObjectStore
• replace the type ListStream[list[ObjectMeta]] with ListStream[Sequence[ObjectMeta]] - this appears problematic with obstore 0.7.0 and mypy.
• minor refactor to existing implementations of .list and .list_prefix in ObjectStore.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.rst
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

❌ Patch coverage is 68.75000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.57%. Comparing base (0019733) to head (e2c31d0).
⚠️ Report is 96 commits behind head on main.

Files with missing lines	Patch %	Lines
src/zarr/storage/_obstore.py	68.75%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3227      +/-   ##
==========================================
- Coverage   59.59%   59.57%   -0.02%     
==========================================
  Files          78       78              
  Lines        8696     8701       +5     
==========================================
+ Hits         5182     5184       +2     
- Misses       3514     3517       +3     

Files with missing lines	Coverage Δ
src/zarr/storage/_obstore.py	60.18% <68.75%> (-0.50%)	⬇️

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[TomAugspurger]

Thanks for the PR. I think if you can merge main the pre-commit issue will be fixed.

[slowjazz]

@TomAugspurger thanks for the review! these failures seem unrelated, what else could I do here?