Combining DISTINCT and `ntext` colums

[tznind]

Describe the bug
If a dataset contains columns of datatype ntext (and probably also text then the DISTINCT keyword is not supported by sql server.

Since RDMP automatically adds DISTINCT to the extraction query it generates this is a problem.

The workaround is to set the DISTINCT Strategy of the extraction source to 'Order By and DISTINCT in memory'.

We should try to detect this condition by looking at the datatypes of the ColumnInfo and either guide the user to that feature or auto enable it for that extraction run.