Support COPY TO for partitioned tables.

Support COPY TO for partitioned tables.

Previously, COPY TO command didn't support directly specifying
partitioned tables so users had to use COPY (SELECT ...) TO variant.

This commit adds direct COPY TO support for partitioned
tables, improving both usability and performance. Performance tests
show it's faster than the COPY (SELECT ...) TO variant as it avoids
the overheads of query processing and sending results to the COPY TO
command.

When used with partitioned tables, COPY TO copies the same rows as
SELECT * FROM table. Row-level security policies of the partitioned
table are applied in the same way as when executing COPY TO on a plain
table.

Author: jian he <jian.universality@gmail.com>
Reviewed-by: vignesh C <vignesh21@gmail.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Melih Mutlu <m.melihmutlu@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Atsushi Torikoshi <torikoshia@oss.nttdata.com>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CACJufxEZt%2BG19Ors3bQUq-42-61__C%3Dy5k2wk%3DsHEFRusu7%3DiQ%40mail.gmail.com