pg_createsubscriber: Introduce module-specific logging functions.

pg_createsubscriber: Introduce module-specific logging functions.

Replace generic pg_log_* calls with report_createsub_log() and
report_createsub_fatal(). This refactor provides the necessary
infrastructure to support logging to external files via the -l option.

These new functions enable the utility to route messages to both the
terminal and a log file based on the logging configuration and verbosity
levels provided by the user.

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Author: Gyan Sreejith <gyan.sreejith@gmail.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CAEqnbaUthOQARV1dscGvB_EsqC-YfxiM6rWkVDHc+G+f4oSUHw@mail.gmail.com