feat: Add arr.gather

[gab23r]

I copy pasted a bunch of code from list.gather. Should we mutualize things into some utility module ?

Part of: #21302

[codecov]

Codecov Report

❌ Patch coverage is 87.84530% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.80%. Comparing base (37cda80) to head (ced99e7).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
...es/polars-ops/src/chunked_array/array/namespace.rs	86.75%	20 Missing ⚠️
crates/polars-plan/src/dsl/function_expr/array.rs	0.00%	1 Missing ⚠️
...polars-plan/src/plans/aexpr/function_expr/array.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25785      +/-   ##
==========================================
- Coverage   80.57%   79.80%   -0.78%     
==========================================
  Files        1764     1764              
  Lines      242704   242931     +227     
  Branches     3042     3044       +2     
==========================================
- Hits       195561   193864    -1697     
- Misses      46361    48285    +1924     
  Partials      782      782              

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[orlp]

This should be a dedicated kernel rather than treating the array as a bunch of Series. Right now I don't see how this is any better than .arr.eval(pl.element().gather(idxs), as_list=True).