Populate FileWriteMetadata.format_metadata with Thrift-serialized Parquet footer for FFI consumers

[qzyu999]

Background

PR #23656 added FileWriteMetadata with a format_metadata: Option<Bytes> field designed to carry format-specific metadata (e.g., Thrift-serialized Parquet FileMetaData). The current implementation sets this to None because the typed ParquetMetaData is already accessible via ParquetSink::written() for Rust consumers.

Proposal

When a concrete FFI use case emerges (e.g., Python via PyO3, or C via datafusion-ffi), serialize the Parquet FileMetaData using Thrift Compact Protocol and populate format_metadata so that non-Rust consumers can access per-file column statistics without re-reading file footers.

Context

• PR: Datasource: Add file_metadata() to DataSink for per-file write metadata #23656
• Review comment: Datasource: Add file_metadata() to DataSink for per-file write metadata #23656 (comment)