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
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0a7ccee8fe9ffaff09ad45f4890a3457c69e80b7

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(+)