logical decoding/replication: new functions pg_ls_logicaldir and pg_ls_replslotdir

Hi,

At times, users want to know what are the files (snapshot and mapping
files) that are available under pg_logical directory and also the
spill files that are under pg_replslot directory and how much space
they occupy. This will help to better know the storage usage pattern
of these directories. Can we have two new functions pg_ls_logicaldir
and pg_ls_replslotdir on the similar lines of pg_ls_logdir,
pg_ls_logdir,pg_ls_tmpdir, pg_ls_archive_statusdir [1]?

[1] - https://www.postgresql.org/docs/devel/functions-admin.html

Regards,
Bharath Rupireddy.