feat: Create literals for datetime/date expressions

[ritchie46]

Much clearer for the optimizer.

[codspeed-hq]

CodSpeed Performance Report

Merging #18184 will improve performances by 42.56%

_{Comparing lit (df1bc35) with main (8111493)}

Summary

⚡ 1 improvements
✅ 36 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	lit	Change
⚡	test_groupby_h2oai_q5	2.6 ms	1.8 ms	+42.56%

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 80.30%. Comparing base (8111493) to head (df1bc35).
Report is 33 commits behind head on main.

Files	Patch %	Lines
crates/polars-plan/src/dsl/functions/temporal.rs	86.44%	8 Missing ⚠️
crates/polars-arrow/src/temporal_conversions.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18184      +/-   ##
==========================================
- Coverage   80.32%   80.30%   -0.02%     
==========================================
  Files        1500     1500              
  Lines      198745   198810      +65     
  Branches     2833     2833              
==========================================
+ Hits       159634   159662      +28     
- Misses      38584    38621      +37     
  Partials      527      527              

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