feat(python): Make the available concat alignment strategies more generic

[alexander-beedie]

Closes #20637.

Simple extension to allow for more generic align options in concat (the concat strategy for "align" is directly associated with the "how" join method, so we can easily use join methods other than "full" - this can result in significant speedups if we know that "align_left" is sufficient/appropriate for our target frames 🚀)

• Extends concat "how" parameters with "align inner", "align_left", and "align_right" (and an explicit "align_full" which is equivalent to the existing/default "align").

• Streamlines the internal concat/align code to use coalesce=True for all join types; no longer have to manually identify/drop the uncoalesced common join cols. Should get a (minor) speedup from this too.

Example

import polars as pl

df1 = pl.DataFrame({"id": [1, 2], "x": [3, 4]})
df2 = pl.DataFrame({"id": [2, 3], "y": [5, 6]})
df3 = pl.DataFrame({"id": [1, 3], "z": [7, 8]})

pl.concat([df1, df2, df3], how="align")  # equivalent to "align_full"
# shape: (3, 4)
# ┌─────┬──────┬──────┬──────┐
# │ id  ┆ x    ┆ y    ┆ z    │
# │ --- ┆ ---  ┆ ---  ┆ ---  │
# │ i64 ┆ i64  ┆ i64  ┆ i64  │
# ╞═════╪══════╪══════╪══════╡
# │ 1   ┆ 3    ┆ null ┆ 7    │
# │ 2   ┆ 4    ┆ 5    ┆ null │
# │ 3   ┆ null ┆ 6    ┆ 8    │
# └─────┴──────┴──────┴──────┘

pl.concat([df1, df2, df3], how="align_left")
# shape: (2, 4)
# ┌─────┬─────┬──────┬──────┐
# │ id  ┆ x   ┆ y    ┆ z    │
# │ --- ┆ --- ┆ ---  ┆ ---  │
# │ i64 ┆ i64 ┆ i64  ┆ i64  │
# ╞═════╪═════╪══════╪══════╡
# │ 1   ┆ 3   ┆ null ┆ 7    │
# │ 2   ┆ 4   ┆ 5    ┆ null │
# └─────┴─────┴──────┴──────┘

pl.concat([df1, df2, df3], how="align_right")
# shape: (2, 4)
# ┌─────┬──────┬──────┬─────┐
# │ id  ┆ x    ┆ y    ┆ z   │
# │ --- ┆ ---  ┆ ---  ┆ --- │
# │ i64 ┆ i64  ┆ i64  ┆ i64 │
# ╞═════╪══════╪══════╪═════╡
# │ 1   ┆ null ┆ null ┆ 7   │
# │ 3   ┆ null ┆ 6    ┆ 8   │
# └─────┴──────┴──────┴─────┘

pl.concat([df1, df2, df3], how="align_inner")
# shape: (0, 4)
# ┌─────┬─────┬─────┬─────┐
# │ id  ┆ x   ┆ y   ┆ z   │
# │ --- ┆ --- ┆ --- ┆ --- │
# │ i64 ┆ i64 ┆ i64 ┆ i64 │
# ╞═════╪═════╪═════╪═════╡
# └─────┴─────┴─────┴─────┘

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.02%. Comparing base (247f0b1) to head (440e847).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/functions/eager.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #20644      +/-   ##
==========================================
- Coverage   79.03%   79.02%   -0.02%     
==========================================
  Files        1557     1557              
  Lines      220528   220554      +26     
  Branches     2510     2514       +4     
==========================================
- Hits       174303   174286      -17     
- Misses      45651    45695      +44     
+ Partials      574      573       -1     

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