Fix issue 791: add LazyFrame sink classes for parquet, csv, ipc, ndjson

[sonalishintre]

Summary

Closes #791

Adds data sink support for Polars LazyFrames, allowing users to write
LazyFrames directly to disk without calling lf.collect() first, which
is more performant.

Changes

• Added PolarsLazyFrameSinkParquet class
• Added PolarsLazyFrameSinkCSV class
• Added PolarsLazyFrameSinkIPC class
• Added PolarsLazyFrameSinkNDJSON class
• Registered all new sinks in register_data_loaders()

How I tested this

Added 4 new tests in tests/plugins/test_polars_lazyframe_extensions.py:

• test_polars_lazyframe_sink_parquet ✅
• test_polars_lazyframe_sink_csv ✅
• test_polars_lazyframe_sink_ipc ✅
• test_polars_lazyframe_sink_ndjson ✅

Each test creates a LazyFrame, sinks it to a temp file, reads it back,
and verifies the data matches.

Notes

• sink_ndjson has no corresponding loader yet as noted in the issue

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Any change in functionality is tested
• New functions are documented (with a description and docstring)

[sonalishintre]

Hi @skrawcz,
this is my first contribution to this project.
I've implemented the LazyFrame sink classes requested in #791
(sink_parquet, sink_csv, sink_ipc, sink_ndjson) with tests for each.
Would appreciate a review when you have time!