Add support for array of arrays

[timvancann]

The structure for an array of arrays as it is currently rendered is not supported by fastavro. This PR fixes this nested structure such that pydantic models with list of lists can be parsed into an avro schema correctly.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.13 🎉

Comparison is base (705dc7c) 90.59% compared to head (70e83c7) 90.73%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
+ Coverage   90.59%   90.73%   +0.13%     
==========================================
  Files           2        2              
  Lines         202      205       +3     
==========================================
+ Hits          183      186       +3     
  Misses         19       19              

Impacted Files	Coverage Δ
src/pydantic_avro/base.py	98.03% <100.00%> (+0.05%)	⬆️

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