feat: Add Series::{first, last, approx_n_unique}

[coastalwhite]

This now implements the aggregations that were already available as DataFrame expressions for Series as well.

Fixes #19073.

[codecov]

Codecov Report

Attention: Patch coverage is 56.33803% with 31 lines in your changes missing coverage. Please review.

Project coverage is 79.76%. Comparing base (f55658b) to head (8fa41ab).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/series/series_trait.rs	63.15%	7 Missing ⚠️
crates/polars-python/src/series/aggregation.rs	46.15%	7 Missing ⚠️
crates/polars-core/src/frame/column/mod.rs	50.00%	4 Missing ⚠️
...s/polars-core/src/series/implementations/binary.rs	0.00%	3 Missing ⚠️
.../polars-core/src/series/implementations/boolean.rs	0.00%	3 Missing ⚠️
...s/polars-core/src/series/implementations/floats.rs	0.00%	3 Missing ⚠️
...s/polars-core/src/series/implementations/string.rs	0.00%	3 Missing ⚠️
py-polars/polars/series/series.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19093      +/-   ##
==========================================
- Coverage   79.77%   79.76%   -0.01%     
==========================================
  Files        1531     1531              
  Lines      208529   208561      +32     
  Branches     2913     2913              
==========================================
+ Hits       166347   166368      +21     
- Misses      41631    41642      +11     
  Partials      551      551              

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[ritchie46]

This one still needs docs entries in the python reference guide. @coastalwhite