pgsql: Don't pass grouped relations to FDWs

Don't pass grouped relations to FDWs

build_grouped_rel() makes a flat copy of the input relation, including
its FDW fields. As a result, when building a grouped join relation
for eager aggregation, add_paths_to_joinrel() called the FDW's
GetForeignJoinPaths for it. postgres_fdw then failed with "Aggref
found where not expected" on the partial Aggrefs in the target list,
and in any case it would have treated a partially aggregated input as
a plain foreign scan.

FDWs are not prepared to handle grouped relations, so clear serverid,
fdwroutine and fdw_private in build_grouped_rel().

Reported-by: Robert Haas <robertmhaas(at)gmail(dot)com>
Author: Richard Guo <guofenglinux(at)gmail(dot)com>
Discussion: https://postgr.es/m/CA+Tgmob7iSM9YkRM44VjUDuaCchW-fY54MV5njpTZTL9uNyV4w@mail.gmail.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/58fca6bdf26f55484c11bf505ae699ec240d60cb

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out | 26 ++++++++++++++++++++++++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 5 +++++
src/backend/optimizer/util/relnode.c | 7 +++++++
3 files changed, 38 insertions(+)