Handle more gracefully checks on select_sql argument of DbToCSVOperator

[augusto-herrmann]

Fix #240

This will:

• handle gracefully more corner cases of values that users pass to select_sql in DbToCSVOperator. Will handle Optional[str | Path] types. If a str, try to determine if it is a file path or a query string.
• tolerate invalid values in int_columns, interpret as pd.NA if it's not possible to convert to integer, instead of generating an exception.

[marcusva86]

reviewed