Add virtual methods to DataTransformerBase for GetBytes, GetChars,
GetData, and GetDataTypeName that properly handle computed columns
by throwing NotSupportedException when MapOrdinal returns -1.
Update TransformingDataReader to delegate these methods to the
transformer instead of directly to the source reader.
Add core transformer infrastructure for the ETL pipeline:
- DataTransformerBase: abstract base class with virtual methods for
field count, names, types, values, ordinals, and null checking
- TransformingDataReader: IDataReader wrapper that delegates to
transformer, enabling on-the-fly data transformations
Joseph Doherty