Add async oindex and vindex methods to AsyncArray

[TomNicholas]

Array has .oindex and .vindex methods, but AsyncArray has no equivalent. This PR adds them. It only adds the get methods, not the set methods, which I thought could be deferred to a follow-up PR.

I want it for pydata/xarray#10327 (comment)

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

Attention: Patch coverage is 57.77778% with 19 lines in your changes missing coverage. Please review.

Project coverage is 59.64%. Comparing base (a27d4d6) to head (102e411).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/zarr/core/indexing.py	50.00%	13 Missing ⚠️
src/zarr/core/array.py	68.42%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3083      +/-   ##
==========================================
- Coverage   59.65%   59.64%   -0.01%     
==========================================
  Files          78       78              
  Lines        8690     8735      +45     
==========================================
+ Hits         5184     5210      +26     
- Misses       3506     3525      +19     

Files with missing lines	Coverage Δ
src/zarr/core/array.py	69.01% <68.42%> (-0.02%)	⬇️
src/zarr/core/indexing.py	68.53% <50.00%> (-0.69%)	⬇️

... and 8 files with indirect coverage changes

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[TomNicholas]

@dcherian suggested making the sync oindex and vindex getitem methods call the new async versions.