feat: Implements arr.n_unique

[reswqa]

Counterpart of list.n_unique in array namespace. This closes #14273.

(After a busy period, finally had time to play with polars 😉)

[codecov]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.35%. Comparing base (705b148) to head (b86789a).
Report is 2 commits behind head on main.

Files	Patch %	Lines
crates/polars-plan/src/dsl/function_expr/array.rs	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15296      +/-   ##
==========================================
+ Coverage   81.32%   81.35%   +0.03%     
==========================================
  Files        1360     1364       +4     
  Lines      176070   176596     +526     
  Branches     2526     2526              
==========================================
+ Hits       143183   143670     +487     
- Misses      32403    32443      +40     
+ Partials      484      483       -1     

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[ritchie46]

Good to see you again on the PR board!