feat: Support non-coalescing joins in default engine

[ritchie46]

closes #14925

Left join still coalesces by default. Would need a breaking release to change the default.

[codecov]

Codecov Report

Attention: Patch coverage is 94.11765% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 80.92%. Comparing base (a3dedb1) to head (a181627).
Report is 4 commits behind head on main.

Files	Patch %	Lines
.../polars-lazy/src/physical_plan/streaming/checks.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16036   +/-   ##
=======================================
  Coverage   80.92%   80.92%           
=======================================
  Files        1385     1385           
  Lines      178242   178233    -9     
  Branches     3050     3050           
=======================================
- Hits       144234   144228    -6     
+ Misses      33520    33516    -4     
- Partials      488      489    +1     

Flag	Coverage Δ
python	74.40% <88.23%> (+<0.01%)	⬆️
rust	78.13% <94.11%> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #16036 will improve performances by 33.88%

_{Comparing join (a181627) with main (5da378f)}

Summary

⚡ 1 improvements
✅ 34 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	join	Change
⚡	test_filter2	2.8 ms	2.1 ms	+33.88%