refactor: Sink DSL and callback for Iceberg

[nameexhaustion]

• Add a SinkType::Iceberg to the DSL, replacing the existing usage of ::PipeWithSchema
• Add a SinkedPathsCallback::IcebergCommit variant, and refactors Iceberg commit to use this callback instead of a separate file-listing

[github-actions]

The uncompressed lib size after this PR is 58.9041 MB.

[codecov]

Codecov Report

❌ Patch coverage is 86.30137% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.19%. Comparing base (efe654e) to head (788e7f1).
⚠️ Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/src/polars/io/iceberg/_sink.py	86.79%	6 Missing and 1 partial ⚠️
.../polars-plan/src/dsl/options/iceberg_sink_state.rs	81.81%	4 Missing ⚠️
crates/polars-utils/src/pl_str.rs	66.66%	3 Missing ⚠️
crates/polars-utils/src/python_function.rs	50.00%	3 Missing ⚠️
crates/polars-plan/src/dsl/options/sink.rs	90.47%	2 Missing ⚠️
crates/polars-plan/src/client/check.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #27258      +/-   ##
==========================================
- Coverage   81.80%   81.19%   -0.61%     
==========================================
  Files        1816     1821       +5     
  Lines      250592   251105     +513     
  Branches     3144     3144              
==========================================
- Hits       204985   203882    -1103     
- Misses      44803    46423    +1620     
+ Partials      804      800       -4     

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