refactor(python): Move dedicated inference code out of io.database executor module

[alexander-beedie]

Some housekeeping, in advance of adding further capabilities (backing-out more accurate dtypes from the underlying db driver module when the cursor description contains a custom type_code).

This finishes consolidating inference code out of the executor module.

[codecov]

Codecov Report

Attention: Patch coverage is 45.83333% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 81.16%. Comparing base (4b94d2f) to head (6d16bc2).
Report is 1 commits behind head on main.

Files	Patch %	Lines
py-polars/polars/io/database/_inference.py	35.00%	10 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15526      +/-   ##
==========================================
- Coverage   81.16%   81.16%   -0.01%     
==========================================
  Files        1362     1362              
  Lines      175015   175009       -6     
  Branches     2533     2532       -1     
==========================================
- Hits       142047   142041       -6     
- Misses      32484    32485       +1     
+ Partials      484      483       -1     

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[stinodego]

Looks fine to me!