[FEA] Stream-preserving constructors for scalar class and its derived classes

**Is your feature request related to a problem? Please describe.**
When the class constructor is implicitly called (when the object is passed-by-value, for instance), the constructor is called with the default stream, and not the CUDA stream passed by the user on which the algorithm is running.
`string_scalar(string_scalar const& other,
rmm::cuda_stream_view stream = cudf::get_default_stream(),
rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());`

**Describe the solution you'd like**
Consider requiring stream to be passed to constructor by making it a non-default argument.
`string_scalar(string_scalar const& other,
rmm::cuda_stream_view stream ,
rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());
`

**Describe alternatives you've considered**
None

**Additional context**
None