chore: Rename Unit to Plain in Parquet reader

[coastalwhite]

This is more like the spec.

[codecov]

Codecov Report

Attention: Patch coverage is 61.53846% with 15 lines in your changes missing coverage. Please review.

Project coverage is 80.41%. Comparing base (1df3b0b) to head (695d285).

Files	Patch %	Lines
...arquet/src/arrow/read/deserialize/binary/nested.rs	0.00%	3 Missing ⚠️
...arrow/read/deserialize/fixed_size_binary/nested.rs	0.00%	3 Missing ⚠️
...quet/src/arrow/read/deserialize/primitive/basic.rs	40.00%	3 Missing ⚠️
...parquet/src/arrow/read/deserialize/binary/basic.rs	0.00%	2 Missing ⚠️
...quet/src/arrow/read/deserialize/binary/decoders.rs	33.33%	2 Missing ⚠️
...arquet/src/arrow/read/deserialize/binview/basic.rs	50.00%	1 Missing ⚠️
...rquet/src/arrow/read/deserialize/binview/nested.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17751   +/-   ##
=======================================
  Coverage   80.40%   80.41%           
=======================================
  Files        1502     1502           
  Lines      197041   197041           
  Branches     2794     2794           
=======================================
+ Hits       158439   158448    +9     
+ Misses      38088    38079    -9     
  Partials      514      514           

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