pgsql: Add SQL functions to monitor the directory contents of replicati

Add SQL functions to monitor the directory contents of replication slots

This commit adds a set of functions able to look at the contents of
various paths related to replication slots:
- pg_ls_logicalsnapdir, for pg_logical/snapshots/
- pg_ls_logicalmapdir, for pg_logical/mappings/
- pg_ls_replslotdir, for pg_replslot/<slot_name>/

These are intended to be used by monitoring tools.  Unlike pg_ls_dir(),
execution permission can be granted to non-superusers.  Roles members of
pg_monitor gain have access to those functions.

Bump catalog version.

Author: Bharath Rupireddy
Reviewed-by: Nathan Bossart, Justin Pryzby
Discussion: https://postgr.es/m/CALj2ACWsfizZjMN6bzzdxOk1ADQQeSw8HhEjhmVXn_Pu+7VzLw@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/1922d7c6e1a74178bd2f1d5aa5a6ab921b3fcd34

Modified Files
--------------
contrib/test_decoding/expected/slot.out      | 21 ++++++++
contrib/test_decoding/sql/slot.sql           |  6 +++
doc/src/sgml/func.sgml                       | 73 ++++++++++++++++++++++++++++
src/backend/catalog/system_functions.sql     | 12 +++++
src/backend/utils/adt/genfile.c              | 44 +++++++++++++++++
src/include/catalog/catversion.h             |  2 +-
src/include/catalog/pg_proc.dat              | 22 +++++++++
src/test/regress/expected/misc_functions.out | 50 +++++++++++++++++++
src/test/regress/sql/misc_functions.sql      | 21 ++++++++
9 files changed, 250 insertions(+), 1 deletion(-)