pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9cd92d1a33699f86aa53d44ab04cc3eb50c18d11

Modified Files
--------------
doc/src/sgml/func.sgml               | 27 ++++++++++++++++
src/backend/catalog/system_views.sql |  4 +++
src/backend/utils/adt/genfile.c      | 62 ++++++++++++++++++++++++++++++++----
src/include/catalog/catversion.h     |  2 +-
src/include/catalog/pg_proc.dat      | 10 ++++++
5 files changed, 97 insertions(+), 8 deletions(-)