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