WIP: Add an eager reader that loads data into RAM

[jni]

Depends on: napari/napari#4182
Depends on: napari/napari#4391

Related feature request: https://forum.image.sc/t/napari-ome-zarr-plugin-slow-navigation/121720

Note: this isn't ready to be merged, because napari doesn't yet support multiple readers for the same file pattern within a single plugin. (It crashes when attempting to open matching files.) However, this should work, but just requires the above issues to be fixed in napari, so I'm putting this here as a proof of concept.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 27.77778% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.02%. Comparing base (2b7e0e9) to head (aa33577).
⚠️ Report is 35 commits behind head on main.

Files with missing lines	Patch %	Lines
napari_ome_zarr/_reader.py	23.52%	13 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (2b7e0e9) and HEAD (aa33577). Click for more details.

HEAD has 3 uploads less than BASE

Flag	BASE (2b7e0e9)	HEAD (aa33577)
	6	3

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #160       +/-   ##
===========================================
- Coverage   86.82%   72.02%   -14.80%     
===========================================
  Files           5        5               
  Lines         516      690      +174     
===========================================
+ Hits          448      497       +49     
- Misses         68      193      +125     

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