This allows table AM to return a native tuple slot even if
VirtualTupleTableSlot is given as an input. Native tuple slots have knowledge
about system attributes, which could be accessed in the future.
table_multi_insert() method already can modify the input 'slots' array.
Discussion: https://postgr.es/m/CAPpHfdurb9ycV8udYqM%3Do0sPS66PJ4RCBM1g-bBpvzUfogY0EA%40mail.gmail.com
Reviewed-by: Matthias van de Meent, Mark Dilger, Pavel Borisov
Reviewed-by: Nikita Malakhov, Japin Li