Update logic of smart_source macro to consider DBT_POPULATE_DEV_TABLES environment variable if it exists

Updated the logic in smart_source macro to populate the tables in the development environment in standard datasets. Add an environment variable DBT_POPULATE_DEV_TABLES and set it to TRUE if the environment variable does not exist it will continue with the default behavior. The DBT_POPULATE_DEV_TABLES is not mandatory it is only required if you want to populate your development environment with actual data using the smart_source macro. The macro assumes the default value of DBT_POPULATE_DEV_TABLES as FALSE if it does not exist