refactor(rust!): prepare for join coalescing argument

[ritchie46]

Prepare for a coalesce: None | bool argument in the join operator. This will default to None and keep the join specific join key coalescing behavior. But users will be able to set the behavior and force/turn off coalescing if needed.

TODO: instead of gathering the redundant column, clone it post-join as this will save 1 column of data.

[codspeed-hq]

CodSpeed Performance Report

Merging #15418 will improve performances by 28.76%

_{Comparing join_coal (21b8416) with main (3bf32f0)}

Summary

⚡ 1 improvements
✅ 12 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	join_coal	Change
⚡	test_filter2	2.9 ms	2.3 ms	+28.76%

[codecov]

Codecov Report

Attention: Patch coverage is 91.93548% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 81.30%. Comparing base (c3f4201) to head (21b8416).
Report is 2 commits behind head on main.

Files	Patch %	Lines
crates/polars-ops/src/frame/join/args.rs	90.00%	2 Missing ⚠️
crates/polars-sql/src/context.rs	0.00%	2 Missing ⚠️
py-polars/src/lazyframe/mod.rs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15418      +/-   ##
==========================================
+ Coverage   81.28%   81.30%   +0.01%     
==========================================
  Files        1381     1381              
  Lines      176849   176876      +27     
  Branches     3042     3043       +1     
==========================================
+ Hits       143756   143809      +53     
+ Misses      32611    32585      -26     
  Partials      482      482              

Flag	Coverage Δ
python	74.77% <80.64%> (+0.03%)	⬆️
rust	78.41% <88.70%> (+0.01%)	⬆️

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