feat: support authenticated footer metadata updates

[kiloOhm]

Summary

• add ParquetFooter.UpdateCustomMetadataAsync overloads for paths and streams
• authenticate existing encrypted or signed plaintext footers before mutation
• preserve the original algorithm, AAD identity, footer mode, and key metadata when re-encrypting or re-signing
• share footer serialization between normal writes and metadata-only rewrites
• cover plaintext, AES-GCM, and AES-GCM-CTR footer updates

Review

No unresolved correctness findings. The implementation prepares the replacement footer before truncation, checks cancellation before mutation, leaves row-group bytes unchanged, and leaves authenticated files byte-for-byte unchanged when the supplied key is invalid.

Validation

• dotnet format src/Parquet.slnx --verify-no-changes ...
• dotnet build src/Parquet.slnx -c Release
• 7/7 focused footer regression tests pass
• 922 self-contained tests pass; 3 skipped
• 5 Parquet-MR interoperability tests require the unavailable ENCRYPTED_PARQUET_INSPECTOR_JAR