Use `stream_arn` object for storage of last returned to the user stream
instead of raw `std::string`. `stream_arn` is used for parsing ARN
incoming from the user, for returning `std::string` was used because of
buggy copy / move operations of `stream_arn`. Those were fixed, so we're
fixing usage as well.
Fixes: SCYLLADB-1241
Closesscylladb/scylladb#29578