feat(metadata): add support for metadata with must_understand (ZEP0009)

[LDeakin]

See ZEP0009 https://zarr.dev/zeps/draft/ZEP0009.html#extension-definitions

[codecov]

Codecov Report

Attention: Patch coverage is 98.18182% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.59%. Comparing base (42321fc) to head (7d010ca).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
zarrs_metadata/src/v3/metadata.rs	98.18%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
+ Coverage   81.56%   81.59%   +0.03%     
==========================================
  Files         179      179              
  Lines       25572    25626      +54     
==========================================
+ Hits        20857    20910      +53     
- Misses       4715     4716       +1     

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