feat: Support per-column nulls_last on sort operations

[alexander-beedie]

Fun one... started looking at implementing NULLS FIRST and NULLS LAST for the SQL interface, and needed to extend the core sort "nulls_last" parameter so that (like "descending") it can take per-column values.

• This PR updates SortMultipleOptions and all related sort functions to handle multi-value "nulls_last".
• Once feat!: Dedicated SQLInterface and SQLSyntax errors #16635 merges I'll add the matching SQL implementation.

Examples

import polars

df = pl.DataFrame({"x": [None, 1, None, 3], "y": [3, 2, None, 1]})
# shape: (4, 2)
# ┌──────┬──────┐
# │ x    ┆ y    │
# │ ---  ┆ ---  │
# │ i64  ┆ i64  │
# ╞══════╪══════╡
# │ null ┆ 3    │
# │ 1    ┆ 2    │
# │ null ┆ null │
# │ 3    ┆ 1    │
# └──────┴──────┘

df.sort("x", "y", nulls_last=True)
# shape: (4, 2)
# ┌──────┬──────┐
# │ x    ┆ y    │
# │ ---  ┆ ---  │
# │ i64  ┆ i64  │
# ╞══════╪══════╡
# │ 1    ┆ 2    │
# │ 3    ┆ 1    │
# │ null ┆ 3    │
# │ null ┆ null │
# └──────┴──────┘

df.sort("x", "y", nulls_last=False)
# shape: (4, 2)
# ┌──────┬──────┐
# │ x    ┆ y    │
# │ ---  ┆ ---  │
# │ i64  ┆ i64  │
# ╞══════╪══════╡
# │ null ┆ null │
# │ null ┆ 3    │
# │ 1    ┆ 2    │
# │ 3    ┆ 1    │
# └──────┴──────┘

df.sort("x", "y", nulls_last=[False,True])
# shape: (4, 2)
# ┌──────┬──────┐
# │ x    ┆ y    │
# │ ---  ┆ ---  │
# │ i64  ┆ i64  │
# ╞══════╪══════╡
# │ null ┆ 3    │
# │ null ┆ null │
# │ 1    ┆ 2    │
# │ 3    ┆ 1    │
# └──────┴──────┘

df.sort("x", "y", nulls_last=[True,False])
# shape: (4, 2)
# ┌──────┬──────┐
# │ x    ┆ y    │
# │ ---  ┆ ---  │
# │ i64  ┆ i64  │
# ╞══════╪══════╡
# │ 1    ┆ 2    │
# │ 3    ┆ 1    │
# │ null ┆ null │
# │ null ┆ 3    │
# └──────┴──────┘

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 81.49%. Comparing base (5974ac7) to head (3fadc78).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/polars-ops/src/series/ops/various.rs	0.00%	6 Missing ⚠️
...es/polars-plan/src/logical_plan/alp/tree_format.rs	0.00%	3 Missing ⚠️
crates/polars-expr/src/expressions/sortby.rs	93.75%	1 Missing ⚠️
py-polars/src/lazyframe/visitor/expr_nodes.rs	0.00%	1 Missing ⚠️
py-polars/src/lazyframe/visitor/nodes.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16639      +/-   ##
==========================================
- Coverage   81.51%   81.49%   -0.02%     
==========================================
  Files        1414     1414              
  Lines      185995   186398     +403     
  Branches     3026     3014      -12     
==========================================
+ Hits       151608   151904     +296     
- Misses      33856    33965     +109     
+ Partials      531      529       -2     

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[ritchie46]

Really nice @alexander-beedie. I wanted this for a long time, but didn't get to it.