pg_createsubscriber: Add -l/--logdir option to redirect output to files.

pg_createsubscriber: Add -l/--logdir option to redirect output to files.

This commit introduces a -l (or --logdir) argument to pg_createsubscriber,
allowing users to specify a directory for log files.

When enabled, a timestamped subdirectory is created within the specified
log directory, containing:

pg_createsubscriber_server.log: Captures logs from the standby server
during its start/stop cycles.
pg_createsubscriber_internal.log: Captures the tool's own internal
diagnostic and progress messages.

This ensures that transient server and utility messages are preserved for
troubleshooting after the subscriber creation process completes or errored
out.

Author: Gyan Sreejith <gyan.sreejith@gmail.com>
Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: vignesh C <vignesh21@gmail.com>
Reviewed-by: Euler Taveira <euler@eulerto.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Reviewed-by: Shlok Kyal <shlok.kyal.oss@gmail.com>
Reviewed-by: Peter Smith <smithpb2250@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CAEqnbaUthOQARV1dscGvB_EsqC-YfxiM6rWkVDHc+G+f4oSUHw@mail.gmail.com