Improve support for legacy vlen codecs and compat with zarr-python

[LDeakin]

I would prefer to see the legacy vlen codecs merged into one for Zarr V3, but this supports what zarr-python v3.0.0 is doing.

[codecov]

Codecov Report

Attention: Patch coverage is 95.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.75%. Comparing base (108cd0b) to head (5044e60).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
zarrs/src/config.rs	87.50%	1 Missing ⚠️
zarrs_metadata/src/v2/array.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
+ Coverage   81.61%   81.75%   +0.13%     
==========================================
  Files         166      166              
  Lines       23556    23571      +15     
==========================================
+ Hits        19226    19270      +44     
+ Misses       4330     4301      -29     

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