fix(python): Fix bool/string usage of "column_totals" parameter in write_excel

[alexander-beedie]

Somewhere along the line the "column_totals" parameter of write_excel stopped working for boolean and string 'func_name' variations (dict/list input was still fine, which is likely why this wasn't noticed earlier), eg:

• column_totals=True
• column_totals="average"

This PR fixes both of these uses, and updates "column_totals" typing (to properly indicate that it supports selectors).

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.50%. Comparing base (e96dd26) to head (3eb13f7).
Report is 2 commits behind head on main.

Files	Patch %	Lines
py-polars/polars/io/spreadsheet/_write_utils.py	66.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17846      +/-   ##
==========================================
+ Coverage   80.49%   80.50%   +0.01%     
==========================================
  Files        1504     1504              
  Lines      197156   197161       +5     
  Branches     2806     2805       -1     
==========================================
+ Hits       158704   158732      +28     
+ Misses      37931    37909      -22     
+ Partials      521      520       -1     

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