pgsql: postgres_fdw: Perform the (ORDERED,NULL) upperrel operations re

postgres_fdw: Perform the (ORDERED, NULL) upperrel operations remotely.

The upper-planner pathification allows FDWs to arrange to push down
different types of upper-stage operations to the remote side.  This
commit teaches postgres_fdw to do it for the (ORDERED, NULL) upperrel,
which is responsible for evaluating the query's ORDER BY ordering.
Since postgres_fdw is already able to evaluate that ordering remotely
for foreign baserels and foreign joinrels (see commit aa09cd242f et al.),
this adds support for that for foreign grouping relations.

Author: Etsuro Fujita
Reviewed-By: Antonin Houska and Jeff Janes
Discussion: https://postgr.es/m/87pnz1aby9.fsf@news-spur.riddles.org.uk

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ffab494a4d4676a71e830b2d7285eeeb5f2b53d7

Modified Files
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contrib/postgres_fdw/deparse.c                 |  28 +-
contrib/postgres_fdw/expected/postgres_fdw.out | 167 +++++------
contrib/postgres_fdw/postgres_fdw.c            | 388 +++++++++++++++++++++++--
contrib/postgres_fdw/postgres_fdw.h            |  12 +-
4 files changed, 457 insertions(+), 138 deletions(-)