option to call udf without schema validation

[uhlarmarek]

Description

Im trying to use SqlContext with custom function registry. Registration of function to registry looks like this:

registry.register(
            "isnull",
            UserDefinedFunction::new(
                "isnull",
                vec![
                    Field::new("id", DataType::Int32),
                    Field::new("value", DataType::Int64),
                ],
                SpecialEq::new(Arc::new(IsNullReturnType::new())),
                IsNull::new(),
            ),
        );

Now it is not possible to use custom function for a series other than "id".
This works:

context
      .execute("SELECT isnull(id, 0) FROM Agents")
      .unwrap()
      .collect()
      .unwrap();

This will throw error:

context
      .execute("SELECT isnull(name, 0) FROM Agents")
      .unwrap()
      .collect()
      .unwrap();

The reason is that a logical expression is created by calling call method on UserDefinedFunction struct. This method validates args against input fields from UDF registration.

It would be very useful to have possibility to call UDF without schema validation using some flag in FunctionOptions struct.