postgres_fdw, dblink: Validate use_scram_passthrough values

Hi,

The use_scram_passthrough option in postgres_fdw and dblink is
intended to accept only boolean values. However, unlike other boolean
options such as keep_connections, its value is not currently
validated.

As a result, invalid values are accepted unexpectedly, for example:

CREATE SERVER test_server
  FOREIGN DATA WRAPPER postgres_fdw
  OPTIONS (use_scram_passthrough 'invalid');

Attached patch updates postgres_fdw and dblink to validate
use_scram_passthrough values and throw an error for invalid input.

Regards,

-- 
Fujii Masao