pgsql: postgres_fdw, dblink: Validate use_scram_passthrough values

postgres_fdw, dblink: Validate use_scram_passthrough values

The use_scram_passthrough option in postgres_fdw and dblink accepts
only boolean values. However, unlike other boolean options such as
keep_connections, its value was not previously validated.

As a result, commands such as
"CREATE SERVER ... OPTIONS (use_scram_passthrough 'invalid')"
could succeed unexpectedly.

This commit updates postgres_fdw and dblink to validate that
use_scram_passthrough is assigned a valid boolean value, and throw an
error for invalid input.

Backpatch to v18, where use_scram_passthrough was introduced.

Author: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Ayush Tiwari <ayushtiwari(dot)slg01(at)gmail(dot)com>
Reviewed-by: Matheus Alcantara <matheusssilv97(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAHGQGwF+-k-Ehsu5W94ZP7GxS3wiBd+mi0PfGTdJ_i2Yr0zR3g@mail.gmail.com
Backpatch-through: 18

Branch
------
REL_18_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e5d019fbdc12281f666a94d3d2cf8ad33ae2006a

Modified Files
--------------
contrib/dblink/dblink.c | 3 +++
contrib/postgres_fdw/option.c | 3 ++-
2 files changed, 5 insertions(+), 1 deletion(-)