feat(writer): Add PositionDeleteFileWriter for writing position delet…

…e files

This commit implements the PositionDeleteFileWriter, completing the delete
operations support in iceberg-rust. Position delete files are used to mark
specific rows for deletion by file path and position (row number).

Key features:
- Implements PositionDeleteFileWriter following the same pattern as
  EqualityDeleteFileWriter
- Supports the two required fields: file_path (field id 2147483546) and
  pos (field id 2147483545)
- Allows optional additional columns from deleted rows for debugging
- Sets content type to PositionDeletes with null sort_order_id as per spec
- Includes comprehensive tests for basic usage, extra columns, and
  multiple batches

This implementation is essential for S3 write operations, enabling proper
DELETE and UPDATE operations on Iceberg tables via both S3Tables and REST
catalogs.

crates/iceberg/src/writer/base_writer/mod.rs

@@ -19,3 +19,4 @@
 
 pub mod data_file_writer;
 pub mod equality_delete_writer;
+pub mod position_delete_writer;