feat: Add 'allow_exact_matches' join_asof'

[ritchie46]

closes #15535
closes #7932

[github-actions]

The uncompressed lib size after this PR is 39.2291 MB.

[codecov]

Codecov Report

Attention: Patch coverage is 82.81250% with 33 lines in your changes missing coverage. Please review.

Project coverage is 79.84%. Comparing base (f3906ec) to head (1b254f9).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-ops/src/frame/join/asof/groups.rs	69.76%	26 Missing ⚠️
crates/polars-ops/src/frame/join/asof/mod.rs	85.41%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #20723    +/-   ##
========================================
  Coverage   79.84%   79.84%            
========================================
  Files        1560     1560            
  Lines      221417   221539   +122     
  Branches     2530     2530            
========================================
+ Hits       176797   176898   +101     
- Misses      44038    44059    +21     
  Partials      582      582            

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