Add pg_get_acl() to get the ACL for a database object

Add pg_get_acl() to get the ACL for a database object

This function returns the ACL for a database object, specified by
catalog OID and object OID.  This is useful to be able to
retrieve the ACL associated to an object specified with a
(class_id,objid) couple, similarly to the other functions for object
identification, when joined with pg_depend or pg_shdepend.

Original idea by Álvaro Herrera.

Bump catalog version.

Author: Joel Jacobson
Reviewed-by: Isaac Morland, Michael Paquier, Ranier Vilela
Discussion: https://postgr.es/m/80b16434-b9b1-4c3d-8f28-569f21c2c102@app.fastmail.com