feat(python): Issue warning when using to_struct() without a list of field names

[nameexhaustion]

ref

• struct.prefix_fields() fails after list.to_struct() with InvalidOperationError #20137

We need to wait until 2.0 to fix the API for this, for now we can add a warning

[ritchie46]

Do we have a milestone 2.0 issue for this?

[nameexhaustion]

Do we have a milestone 2.0 issue for this?

I made a draft here, it hasn't been reviewed yet though

• Re-work API for list.to_struct() #19525

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.58%. Comparing base (3fb10e8) to head (da68a12).
Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/expr/list.py	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20158   +/-   ##
=======================================
  Coverage   79.58%   79.58%           
=======================================
  Files        1563     1564    +1     
  Lines      217417   217480   +63     
  Branches     2472     2473    +1     
=======================================
+ Hits       173023   173078   +55     
- Misses      43826    43833    +7     
- Partials      568      569    +1     

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