feat: add dictionary_columns to scan API for memory-efficient string reads

[tanmayrauth]

Exposes dictionary_columns: tuple[str, ...] | None = None on Table.scan() and DataScan, threading it through to PyArrow's ParquetFileFormat so that named columns are read as DictionaryArray instead of plain large_utf8. This dramatically reduces memory usage for high-cardinality repeated JSON/string columns (issue #3168) and addresses the general scan parameter extensibility request (issue #3170).

Key implementation details:

• ORC files are guarded — dictionary_columns is only passed for Parquet
• ArrowScan.to_table() rebuilds the Arrow schema with dict types before the empty-table fast-path so schema is consistent regardless of row count
• DataScan.to_arrow_batch_reader() rebuilds target_schema with dict types to prevent .cast() from silently decoding DictionaryArray back to plain string
• DataScan.__init__ declares and stores the param so TableScan.update() (which uses inspect.signature) preserves it across scan copies

Fixes #3168, closes #3170

Rationale for this change

Are these changes tested? Yes

Are there any user-facing changes? No