Fix GUC check_hook validation for synchronized_standby_slots.

Previously, the check_hook for synchronized_standby_slots attempted to
validate that each specified slot existed and was physical. However, these
checks were not performed during server startup. As a result, if users
configured non-existent slots before startup, the misconfiguration would
go undetected initially. This could later cause parallel query failures,
as newly launched workers would detect the issue and raise an ERROR.

This patch improves the check_hook by validating the syntax and format of
slot names. Validation of slot existence and type is deferred to the WAL
sender process, aligning with the behavior of the check_hook for
primary_slot_name.

Reported-by: Fabrice Chapuis <fabrice636861@gmail.com>
Author: Shlok Kyal <shlok.kyal.oss@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Ashutosh Sharma <ashu.coek88@gmail.com>
Reviewed-by: Rahila Syed <rahilasyed90@gmail.com>
Backpatch-through: 17, where it was introduced
Discussion: https://postgr.es/m/CAA5-nLCeO4MQzWipCXH58qf0arruiw0OeUc1+Q=Z=4GM+=v1NQ@mail.gmail.com

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