dblink: SCRAM authentication pass-through

dblink: SCRAM authentication pass-through

This enables SCRAM authentication for dblink (using dblink_fdw) when
connecting to a foreign server without having to store a plain-text
password on user mapping options

This uses the same approach as it was implemented for postgres_fdw in
commit 761c79508e7.  (It also contains the equivalent of the
subsequent fixes 76563f88cfb and d2028e9bbc1.)

Author: Matheus Alcantara <mths.dev@pm.me>
Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/CAFY6G8ercA1KES%3DE_0__R9QCTR805TTyYr1No8qF8ZxmMg8z2Q%40mail.gmail.com