feat: Create literals in 'datetime' expression

[ritchie46]

The optimizer understands those better.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 80.32%. Comparing base (7b3ab69) to head (02d669e).
Report is 36 commits behind head on main.

Files	Patch %	Lines
crates/polars-plan/src/dsl/functions/temporal.rs	87.50%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18182      +/-   ##
==========================================
+ Coverage   80.31%   80.32%   +0.01%     
==========================================
  Files        1499     1499              
  Lines      198653   198729      +76     
  Branches     2833     2833              
==========================================
+ Hits       159545   159627      +82     
+ Misses      38581    38575       -6     
  Partials      527      527              

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