fix(metadata): Make AdditionalField public and permit any JSON type

[LDeakin]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 94.18605% with 5 lines in your changes missing coverage. Please review.

Project coverage is 82.14%. Comparing base (e55b56f) to head (d48bbde).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
zarrs_metadata/src/v3/metadata.rs	87.80%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #144      +/-   ##
==========================================
+ Coverage   82.12%   82.14%   +0.01%     
==========================================
  Files         173      173              
  Lines       24528    24582      +54     
==========================================
+ Hits        20144    20192      +48     
- Misses       4384     4390       +6     

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