refactor(rust): Remove top-level metadata from ArrowSchema

[nameexhaustion]

Preparation for unifying schemas to struct Schema<D>

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.84%. Comparing base (f5edd91) to head (3840ba3).
Report is 44 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-arrow/src/io/ipc/read/file_async.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18527      +/-   ##
==========================================
- Coverage   79.85%   79.84%   -0.02%     
==========================================
  Files        1501     1501              
  Lines      201865   201823      -42     
  Branches     2868     2868              
==========================================
- Hits       161202   161144      -58     
- Misses      40117    40133      +16     
  Partials      546      546              

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