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

All modified and coverable lines are covered by tests ✅

Project coverage is 94.76%. Comparing base (bbdefac) to head (38aa2eb).
Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3227      +/-   ##
==========================================
+ Coverage   94.47%   94.76%   +0.28%     
==========================================
  Files          78       78              
  Lines        8604     8647      +43     
==========================================
+ Hits         8129     8194      +65     
+ Misses        475      453      -22     

Files with missing lines	Coverage Δ
src/zarr/storage/_obstore.py	93.83% <100.00%> (-0.34%)	⬇️

... and 25 files with indirect coverage changes

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