Add pg_ls_summariesdir().

Add pg_ls_summariesdir().

This function returns the name, size, and last modification time of
each regular file in pg_wal/summaries.  This allows administrators
to grant privileges to view the contents of this directory without
granting privileges on pg_ls_dir(), which allows listing the
contents of many other directories.  This commit also gives the
pg_monitor predefined role EXECUTE privileges on the new
pg_ls_summariesdir() function.

Bumps catversion.

Author: Yushi Ogiwara
Reviewed-by: Michael Paquier, Fujii Masao
Discussion: https://postgr.es/m/a0a3af15a9b9daa107739eb45aa9a9bc%40oss.nttdata.com