warning or some kind of status indication for DuckDB conversions into duckdb/parquet

[e-kotov]

Until progress bar is implemented for the R client for DuckDB (issue here duckdb/duckdb-r#199), provide some indication of how long the conversion might take. Without progress bar, the only way for the user to see the data conversion progress is to go to the output folder and update the file size of the duckdb file or folder with parquet files.