ATS: Use default CLR when field does not exist on server

When field does not exist on server but does in POCO, use the default CLR type.
This looks like pretty general purpose functionality. Can you add a TODO to consider if this should live somewhere else or whether there is something in the framework that we use? (I know k and desktop .NET  implement Convert.ChangeType, but it is not portable and throws when the conversion isn't possible).