dblink: Give user mapping precedence for use_scram_passthrough

dblink: Give user mapping precedence for use_scram_passthrough

Commit 97f6fc10fff changed postgres_fdw so that user-mapping settings
override foreign server settings for use_scram_passthrough. This commit
applies the same behavior to dblink.

Backpatch to v18, where use_scram_passthrough was introduced.

Author: Matheus Alcantara <matheusssilv97@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwEJ8rZjmbOvCicyr4vbuLio082bNTde0WNoSWaWr9wVcg@mail.gmail.com
Backpatch-through: 18