Updated the generation of DataTable row field data to work with other value types

[logiclrd]

DataTableGenerator.cs contains logic to customize the generation of value types by TypeCode value, and falls back on a generated proxy to context.Generate<T> for unrecognized types. The initial implementation of this includes an unnecessary constraint that T be a reference type. This means that columns of types like DateTimeOffset will fail.

This PR:

• Updates the automated testing to trigger this problem.
• Removes the constraint on the proxy type, allowing generic context.Generate<T> calls with value types.