csharp/adbc/ado.net: better mapping for structured types

In the ADO.NET-compatible wrapper for ADBC, lists of scalars are already being returned as IReadOnlyList. Structs should map to something like IDataRecord and lists of structs to either IReadOnlyList or DbDataReader. These types should be accurately reflected when returning the schema. (Today, a LIST of INT32 is incorrectly returned as an INT32.)

Consider adding an option or a configurable mapper so that structured types can easily be converted to JSON.