pgsql: 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(dot)dev(at)pm(dot)me>
Reviewed-by: Jacob Champion <jacob(dot)champion(at)enterprisedb(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/CAFY6G8ercA1KES%3DE_0__R9QCTR805TTyYr1No8qF8ZxmMg8z2Q%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3642df265d09779443a9f44f5cb873df40974e89

Modified Files
--------------
contrib/dblink/Makefile | 1 +
contrib/dblink/dblink.c | 184 ++++++++++++++++++++++++++-
contrib/dblink/meson.build | 5 +
contrib/dblink/t/001_auth_scram.pl | 254 +++++++++++++++++++++++++++++++++++++
doc/src/sgml/dblink.sgml | 21 ++-
doc/src/sgml/postgres-fdw.sgml | 4 +-
6 files changed, 457 insertions(+), 12 deletions(-)