Add ability to insert fields from source struct in ExtractNewRecordState SMT

We have a use case that requires passing the Postgres LSN through to a sink, but also want to flatten the events produced by the Postgres connector. Similar to the __deleted field added by the ExtractNewRecordState transform, it would be great if we could specify fields from the source struct in the event to be added as metadata fields (prefixed with __) to the flattened record.