feat: Add pl.concat_arr to concatenate columns into an Array column

[nameexhaustion]

Fixes #18090

Allows users to concatenates one or more input columns into a fixed-width array column. The input columns are required to have dtypes of either Array(<Dtype>, width) or <Dtype>.

[nameexhaustion]

Added fn horizontal_flatten_unchecked under polars-compute, which performs the flatten operation that creates the values array of the result. This is called from fn concat_arr under polars-ops after it checks the safety conditions.

[codecov]

Codecov Report

Attention: Patch coverage is 79.94580% with 74 lines in your changes missing coverage. Please review.

Project coverage is 79.43%. Comparing base (426cf27) to head (e710547).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...rates/polars-compute/src/horizontal_flatten/mod.rs	60.28%	56 Missing ⚠️
crates/polars-plan/src/dsl/function_expr/array.rs	81.81%	8 Missing ⚠️
crates/polars-ops/src/series/ops/concat_arr.rs	93.02%	6 Missing ⚠️
...s/polars-compute/src/horizontal_flatten/struct_.rs	94.59%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #19881    +/-   ##
========================================
  Coverage   79.43%   79.43%            
========================================
  Files        1551     1554     +3     
  Lines      215222   215591   +369     
  Branches     2452     2452            
========================================
+ Hits       170954   171265   +311     
- Misses      43710    43768    +58     
  Partials      558      558            

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