feat: Improve GroupsProxy/GroupsPosition to be sliceable and cheaply cloneable

[ritchie46]

Preparation for #20669

[github-actions]

The uncompressed lib size after this PR is 38.9850 MB.

[codecov]

Codecov Report

Attention: Patch coverage is 80.16949% with 117 lines in your changes missing coverage. Please review.

Project coverage is 78.99%. Comparing base (87baf86) to head (9a33de7).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-core/src/frame/group_by/position.rs	89.60%	13 Missing ⚠️
crates/polars-expr/src/expressions/slice.rs	62.50%	9 Missing ⚠️
...rs-core/src/frame/group_by/aggregations/boolean.rs	27.27%	8 Missing ⚠️
crates/polars-core/src/series/series_trait.rs	20.00%	8 Missing ⚠️
crates/polars-expr/src/expressions/mod.rs	79.41%	7 Missing ⚠️
...polars-core/src/frame/group_by/aggregations/mod.rs	84.61%	6 Missing ⚠️
...s/polars-core/src/series/implementations/binary.rs	0.00%	5 Missing ⚠️
...s-core/src/frame/group_by/aggregations/agg_list.rs	71.42%	4 Missing ⚠️
.../polars-core/src/series/implementations/boolean.rs	63.63%	4 Missing ⚠️
crates/polars-expr/src/expressions/window.rs	78.94%	4 Missing ⚠️
... and 25 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20673      +/-   ##
==========================================
+ Coverage   78.85%   78.99%   +0.14%     
==========================================
  Files        1558     1559       +1     
  Lines      221055   221204     +149     
  Branches     2527     2527              
==========================================
+ Hits       174318   174751     +433     
+ Misses      46159    45876     -283     
+ Partials      578      577       -1     

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