feat(python): Add "drop_empty_cols" parameter for read_excel and read_ods

[alexander-beedie]

Closes #20376.

Complements the existing "drop_empty_rows" parameter. Note that the default engine calamine does not currently have an option we can use for this, but it looks like one is planned; once available we can hook in to it. In the meantime this parameter has been validated for use with the slower openpyxl and xlsx2csv engines.

Also:

• fixes an issue where columns with the same name would not be loaded when using the openpyxl engine (the rightmost one would "win"; now they are all loaded and get deduplicated with an incrementing suffix (eg: "col", "col0", "col1" ...)
• fixes a homedir expansion bug inadvertently introduced in feat(python): Support loading data from multiple Excel/ODS workbooks #20404.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.01%. Comparing base (f213c36) to head (67343f5).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/io/spreadsheet/functions.py	84.21%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20430      +/-   ##
==========================================
- Coverage   79.02%   79.01%   -0.01%     
==========================================
  Files        1562     1562              
  Lines      220163   220177      +14     
  Branches     2486     2488       +2     
==========================================
+ Hits       173973   173981       +8     
- Misses      45617    45623       +6     
  Partials      573      573              

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