feat: Add nested SQL join support

[wangxiaoying]

fix #18005

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.50%. Comparing base (316d621) to head (44fa2fd).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/polars-sql/src/context.rs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18006      +/-   ##
==========================================
+ Coverage   80.48%   80.50%   +0.01%     
==========================================
  Files        1496     1496              
  Lines      196646   196734      +88     
  Branches     2817     2817              
==========================================
+ Hits       158274   158378     +104     
+ Misses      37851    37835      -16     
  Partials      521      521              

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[ritchie46]

Thanks!