refactor: extract legacy codec interfaces into zarr-codec

[d-v-b]

Summary

AI-written creation of a zarr_codec subpackage that contains our codec APIs. That subpackage would be published and versioned separately from zarr. All current zarr functionality is accessible by importing stuff from the zarr_codec.legacy module.

This PR does not change zarr. Making zarr depend on zarr-codec would happen later, after we stand up the package. This will allow us to develop a new codec API while keeping our old one available indefinitely.

The original AI-written PR is here: d-v-b#326

For reviewers

we can bikeshed the layout of zarr-codec.legacy, or discuss the motivation for subpackaging the codec API. In case you want to chat about this direction more conversationally, I started a Zulip thread#Zarr-Python > subpackaging store and codec APIs.

See also

• fully separate sync and async internal APIs #4217
• how to integrate subpackages #4317
• zarr v3 scale-offset codec, and should the codec API be defined upstream #3867

Author attestation

• I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.23%. Comparing base (9c29a0d) to head (cfbe662).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4319      +/-   ##
==========================================
+ Coverage   94.21%   94.23%   +0.01%     
==========================================
  Files          92       92              
  Lines       12871    12880       +9     
==========================================
+ Hits        12127    12137      +10     
+ Misses        744      743       -1     

see 2 files with indirect coverage changes

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