feat: Add per partition sort and finish callback to sinks

[coastalwhite]

This adds 2 items to partition sinks

• per_partition_sort_by which sorts a partition before writing by certain expressions
• finish_callback which gets called with information about all the written files in the partition

These are needed for better Iceberg support.

[codecov]

Codecov Report

Attention: Patch coverage is 72.63158% with 130 lines in your changes missing coverage. Please review.

Project coverage is 80.69%. Comparing base (622d792) to head (180d7c2).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...lars-stream/src/nodes/io_sinks/partition/parted.rs	0.00%	45 Missing ⚠️
crates/polars-plan/src/dsl/options/sink.rs	27.11%	43 Missing ⚠️
crates/polars-plan/src/plans/conversion/mod.rs	0.00%	10 Missing ⚠️
py-polars/polars/io/partition.py	69.23%	6 Missing and 2 partials ⚠️
...lars-stream/src/nodes/io_sinks/partition/by_key.rs	78.78%	7 Missing ⚠️
crates/polars-python/src/lazyframe/sink.rs	77.77%	6 Missing ⚠️
crates/polars-stream/src/nodes/io_sinks/metrics.rs	94.00%	6 Missing ⚠️
.../polars-stream/src/nodes/io_sinks/partition/mod.rs	93.18%	3 Missing ⚠️
...rs-stream/src/nodes/io_sinks/partition/max_size.rs	97.91%	1 Missing ⚠️
crates/polars-stream/src/physical_plan/to_graph.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #22789    +/-   ##
========================================
  Coverage   80.69%   80.69%            
========================================
  Files        1671     1672     +1     
  Lines      221440   221883   +443     
  Branches     2791     2798     +7     
========================================
+ Hits       178682   179051   +369     
- Misses      42093    42165    +72     
- Partials      665      667     +2     

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