pgsql: postgres_fdw: set search_path to 'pg_catalog' while deparsing co

postgres_fdw: set search_path to 'pg_catalog' while deparsing constants.

The motivation for this is to ensure successful transmission of the
values of constants of regconfig and other reg* types. The remote
will be reading them with search_path = 'pg_catalog', so schema
qualification is necessary when referencing objects in other schemas.

Per bug #17483 from Emmanuel Quincerot. Back-patch to all supported
versions. (There's some other stuff to do here, but it's less
back-patchable.)

Discussion: https://postgr.es/m/1423433.1652722406@sss.pgh.pa.us

Branch
------
REL_12_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/288e499ba817ccb364b9bc826a4470f03ca23b72

Modified Files
--------------
contrib/postgres_fdw/expected/postgres_fdw.out | 20 ++++++++++++++++++++
contrib/postgres_fdw/postgres_fdw.c | 8 ++++++++
contrib/postgres_fdw/sql/postgres_fdw.sql | 9 +++++++++
3 files changed, 37 insertions(+)