Add pg_ls_tmpdir function

Add pg_ls_tmpdir function

This lists the contents of a temporary directory associated to a given
tablespace, useful to get information about on-disk consumption caused
by temporary files used by a session query.  By default, pg_default is
scanned, and a tablespace can be specified as argument.

This function is intended to be used by monitoring tools, and, unlike
pg_ls_dir(), access to them can be granted to non-superusers so that
those monitoring tools can observe the principle of least privilege.
Access is also given by default to members of pg_monitor.

Author: Nathan Bossart
Reviewed-by: Laurenz Albe
Discussion: https://postgr.es/m/92F458A2-6459-44B8-A7F2-2ADD3225046A@amazon.com