refactor(rust): Minor refactor of Rust any value constructors

[stinodego]

Just extracting some logic into functions and moving some stuff around.

[codecov]

Codecov Report

Attention: Patch coverage is 83.97790% with 29 lines in your changes are missing coverage. Please review.

Project coverage is 81.04%. Comparing base (f08914b) to head (b9829c4).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/polars-core/src/series/any_value.rs	83.97%	29 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15077      +/-   ##
==========================================
- Coverage   81.04%   81.04%   -0.01%     
==========================================
  Files        1338     1338              
  Lines      173771   173789      +18     
  Branches     2456     2456              
==========================================
+ Hits       140832   140839       +7     
- Misses      32471    32482      +11     
  Partials      468      468              

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