Support source/sink for plain Parquet/ORC/Avro Tables

Supporting plain Parquet/ORC/Avro (partitioned as well as unpartitioned) may be useful for "upgrading" legacy data to table formats. Sink may be useful for exporting a specific snapshot for interoperability reasons.

This feature is lower priority, as Iceberg/Delta etc have native support for metadata-only conversions and offer Spark procedures.

No files, tests, or entry points are named. Start by reading the repository's existing conversion paths and how Iceberg and Delta support is organized, then determine the source and sink boundaries for plain Parquet, ORC, and Avro. Done means supporting both partitioned and unpartitioned tables for source and sink operations.

Written by the indexing model from the issue text.