pgsql: Allow table AM tuple_insert() method to return the different slo

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c35a3fb5e067fc95f13206418e3785d2cb059da1

Modified Files
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src/backend/access/heap/heapam_handler.c |  4 +++-
src/backend/executor/nodeModifyTable.c   |  6 +++---
src/include/access/tableam.h             | 21 ++++++++++++---------
3 files changed, 18 insertions(+), 13 deletions(-)