pgsql: dblink: Reject use_scram_passthrough on foreign-data wrappers

dblink: Reject use_scram_passthrough on foreign-data wrappers

Previously, dblink accepted the use_scram_passthrough option on
foreign-data wrappers via ALTER FOREIGN DATA WRAPPER dblink_fdw
OPTIONS, even though the setting had no effect there.

use_scram_passthrough should be only meaningful for foreign servers
and user mappings, so this commit updates dblink to accept the option
only in those contexts.

Backpatch to v18, where use_scram_passthrough was introduced.

Author: Matheus Alcantara <matheusssilv97(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAHGQGwEJ8rZjmbOvCicyr4vbuLio082bNTde0WNoSWaWr9wVcg@mail.gmail.com
Backpatch-through: 18

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/e2b8813403b19609a3f194b1d4cdb88df5aa4c7d

Modified Files
--------------
contrib/dblink/dblink.c | 11 +++++++++--
contrib/dblink/expected/dblink.out | 5 +++++
contrib/dblink/sql/dblink.sql | 4 ++++
3 files changed, 18 insertions(+), 2 deletions(-)