pgsql: Ensure that foreign scans with lateral refs are plannedcorrectl

Ensure that foreign scans with lateral refs are planned correctly.

As reported in bug #15613 from Srinivasan S A, file_fdw and postgres_fdw
neglected to mark plain baserel foreign paths as parameterized when the
relation has lateral_relids.  Other FDWs have surely copied this mistake,
so rather than just patching those two modules, install a band-aid fix
in create_foreignscan_path to rectify the mistake centrally.

Although the band-aid is enough to fix the visible symptom, correct
the calls in file_fdw and postgres_fdw anyway, so that they are valid
examples for external FDWs.

Also, since the band-aid isn't enough to make this work for parameterized
foreign joins, throw an elog(ERROR) if such a case is passed to
create_foreignscan_path.  This shouldn't pose much of a problem for
existing external FDWs, since it's likely they aren't trying to make such
paths anyway (though some of them may need a defense against joins with
lateral_relids, similar to the one this patch installs into postgres_fdw).

Add some assertions in relnode.c to catch future occurrences of the same
error --- in particular, as backstop against core-code mistakes like the
one fixed by commit bdd9a99aa.

Discussion: https://postgr.es/m/15613-092be1be9576c728@postgresql.org

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e3101a0317a9171ced56b91e30d8929094d182eb

Modified Files
--------------
contrib/file_fdw/file_fdw.c                    |  6 ++-
contrib/postgres_fdw/expected/postgres_fdw.out | 56 ++++++++++++++++++++++++++
contrib/postgres_fdw/postgres_fdw.c            | 18 +++++++--
contrib/postgres_fdw/sql/postgres_fdw.sql      | 26 ++++++++++++
src/backend/optimizer/util/pathnode.c          | 21 ++++++++++
src/backend/optimizer/util/relnode.c           |  9 +++++
6 files changed, 131 insertions(+), 5 deletions(-)