[FEA] Scalar factory functions do not let you provide a validity bool.

Scalars can be null and contain a validity bool. The constructors for the various scalars take an `is_valid` parameter for this purpose. But the factory functions themselves don't let you specify one in the call. Seems like an oversight. This applies to all scalar types.
```
struct_scalar(table&& data,
bool is_valid = true,
rmm::cuda_stream_view stream = rmm::cuda_stream_default,
rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());
```

```
std::unique_ptr make_struct_scalar(
table_view const& data,
rmm::cuda_stream_view stream = rmm::cuda_stream_default,
rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());
```