feat(python): Allow iterable of frames as input to align_frames

[alexander-beedie]

Closes #21133.

Non-breaking enhancement. Allows for list/iterable input of frames in addition to the current positional input, enabling pl.align_frames and pl.concat to be called in a consistent fashion.

Example

import polars as pl

df1 = pl.DataFrame({"x": [3,4,5], "y": [5,8,9]})
df2 = pl.DataFrame({"x": [2,5,4], "y": [3,8,9]})

Current

pl.align_frames(df1, df2, on="x")

New

pl.align_frames([df1, df2], on="x")

(additive; does not replace current calling convention)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.88%. Comparing base (f69768d) to head (211c73b).
Report is 60 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21209      +/-   ##
==========================================
- Coverage   79.88%   79.88%   -0.01%     
==========================================
  Files        1593     1593              
  Lines      227803   227807       +4     
  Branches     2600     2602       +2     
==========================================
- Hits       181989   181978      -11     
- Misses      45217    45232      +15     
  Partials      597      597              

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