feat: make async_array public

[ilan-gold]

Given that a lot of people are using the private Array._async_array, I think it should be documented and put in a public namespace. This PR specifically does not make zarr.core.sync.sync public because I'm not 100% sure it's necessary in the day-and-age of obstore but also because I can definitely see arguments why a better API might be useful, certain one that does:

some_batching_api(my_sync_array.async_array.getitem(...), my_other_sync_array.async_array.getitem(...))

instead of

zarr.core.sync(asyncio.gather(my_sync_array.async_array.getitem(...), my_other_sync_array.async_array.getitem(...)))

Another reason I think making this public is because you may want to batch together these low level requests just because of the way people's APIs are structured rather than forcing everything to go through something like

some_batching_api(sync_arrays, selections)

Definitely open to opinions on this! Marked as draft for now!

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/*.md
• 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 91.11111% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.76%. Comparing base (b3e9aed) to head (0600057).

Files with missing lines	Patch %	Lines
src/zarr/core/array.py	91.11%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3556      +/-   ##
==========================================
- Coverage   61.77%   61.76%   -0.01%     
==========================================
  Files          85       85              
  Lines       10165    10168       +3     
==========================================
+ Hits         6279     6280       +1     
- Misses       3886     3888       +2     

Files with missing lines	Coverage Δ
src/zarr/core/array.py	68.51% <91.11%> (-0.12%)	⬇️

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