remove chunk encoding from config

[d-v-b]

This PR removes the default chunk encoding parameters from the config.

Prior to the data types refactor, when we inferred the chunk encoding automatically, we categorized the array data type into one of a few categories like "numeric", "string", "bytes". These categories were created before we added datetimes and fixed-length strings, and they don't really make sense after those data types were added:

• "string" and "bytes" are not real categories, because they do not distinguish between fixed length and variable length bytes / strings. The fixed-length types are basically "numeric", but the variable length types must use the vlen utf8 and vlen bytes codecs.
• datetime data, fixed-length strings, and structured data types are not obviously "numeric"

It's not really possible to preserve the old categories in the config now that we have more data types that resist categorization. I don't think it's really possible break the data types down into a small number of general categories, and it's less possible / attractive to put this categorization logic in our config.

I think a better solution is for zarr python to choose sane defaults; when users want something other than the default, they specify it in the function call. In practice these sane defaults are the same codecs for all data types except the variable-length data types, which get special treatment (data types with endianness in zarr v3 also get a bytes codec with an endianness parameter). That's what I added in this PR. The default chunk encoding parameters each come from functions that are inaccessible to configuration.

I did a quick github survey to find users who might be using the zarr config to change the default codecs, and I could only find 2: @TomAugspurger uses the config here and @dcangst in here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.76%. Comparing base (ded59d9) to head (1f8c88b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3228      +/-   ##
==========================================
- Coverage   94.76%   94.76%   -0.01%     
==========================================
  Files          78       78              
  Lines        8642     8672      +30     
==========================================
+ Hits         8190     8218      +28     
- Misses        452      454       +2     

Files with missing lines	Coverage Δ
src/zarr/core/array.py	98.52% <100.00%> (+0.05%)	⬆️
src/zarr/core/config.py	83.33% <100.00%> (-16.67%)	⬇️

... and 2 files with indirect coverage changes

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[d-v-b]

closes #3104

[normanrz]

I think that makes sense!