Allow table AM tuple_insert() method to return the different slot

Allow table AM tuple_insert() method to return the different slot

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