refactor: Implement nested row encoding / decoding

[coastalwhite]

This adds row encoding and decoding for nested types.

It is now possible to properly roundtrip pl.List, pl.Array and pl.Struct including their outer and inner validities through the row encoding en decoding.

Fixes #10747.

[codecov]

Codecov Report

Attention: Patch coverage is 86.45598% with 120 lines in your changes missing coverage. Please review.

Project coverage is 79.48%. Comparing base (426cf27) to head (2e5d2c4).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-row/src/encode.rs	83.38%	99 Missing ⚠️
crates/polars-row/src/decode.rs	91.46%	18 Missing ⚠️
crates/polars-row/src/variable.rs	92.59%	2 Missing ⚠️
...ates/polars-arrow/src/array/fixed_size_list/mod.rs	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19874      +/-   ##
==========================================
+ Coverage   79.43%   79.48%   +0.05%     
==========================================
  Files        1551     1554       +3     
  Lines      215222   216124     +902     
  Branches     2452     2452              
==========================================
+ Hits       170954   171787     +833     
- Misses      43710    43780      +70     
+ Partials      558      557       -1     

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