Add pg_ownerships and pg_privileges system views

Hi hackers,

Here is an attempt to revive this patch from 2021-2022, that has been ready now
for a while, thanks to pg_get_acl() function that was committed in
4564f1c and d898665.

I've renamed the $subject of the email thread, to match the commitfest entry:
https://commitfest.postgresql.org/50/5033/

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Add pg_ownerships and pg_privileges system views.

These new views provide a more accessible and user-friendly way to retrieve
information about object ownerships and privileges.

The view pg_ownerships provides access to information about object ownerships.

The view pg_privileges provides access to information about explicitly
granted privileges on database objects. The special grantee value "-" means
the privilege is granted to PUBLIC.

Example usage:

CREATE ROLE alice;
CREATE ROLE bob;
CREATE ROLE carol;

CREATE TABLE alice_table ();
ALTER TABLE alice_table OWNER TO alice;
REVOKE ALL ON alice_table FROM alice;
GRANT SELECT ON alice_table TO bob;

CREATE TABLE bob_table ();
ALTER TABLE bob_table OWNER TO bob;
REVOKE ALL ON bob_table FROM bob;
GRANT SELECT, UPDATE ON bob_table TO carol;

SELECT * FROM pg_ownerships ORDER BY owner;

classid | objid | objsubid | type | schema | name | identity | owner
----------+-------+----------+-------+--------+-------------+--------------------+-------
pg_class | 16388 | 0 | table | public | alice_table | public.alice_table | alice
pg_class | 16391 | 0 | table | public | bob_table | public.bob_table | bob
(2 rows)

SELECT * FROM pg_privileges ORDER BY grantee;

classid | objid | objsubid | type | schema | name | identity | grantor | grantee | privilege_type | is_grantable
----------+-------+----------+-------+--------+-------------+--------------------+---------+---------+----------------+--------------
pg_class | 16388 | 0 | table | public | alice_table | public.alice_table | alice | bob | SELECT | f
pg_class | 16391 | 0 | table | public | bob_table | public.bob_table | bob | carol | SELECT | f
pg_class | 16391 | 0 | table | public | bob_table | public.bob_table | bob | carol | UPDATE | f
(3 rows)

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Recap:

During the work on this, the need for a pg_get_acl() function was identified.

Thanks to feedback from Peter Eisentraut, the view "pg_permissions" was renamed
to "pg_privileges", since "permissions" is not an SQL term.

David Fetter:
> +1 for both this and the ownerships view.

Joe Conway:
> While this is interesting and probably useful for troubleshooting, it does not
> provide the complete picture if what you care about is something like "what
> stuff can joel do in my database".
>
> The reasons for this include default grants to PUBLIC and role membership, and
> even that is convoluted by INHERIT/NOINHERIT role attributes.

Chapman Flack expressed interest in reviewing the patch, but at that time
the pg_get_acl() had not yet been committed and the view not been renamed.

Michael Paquier alerted me CF bot had been red, and the patch was rebased.

/Joel