Support EXCEPT for TABLES IN SCHEMA publications

Hi hackers,

Following earlier work to support EXCEPT for FOR ALL TABLES [1]
publications, starting this thread to extend the same capability to
schema-level publications (TABLES IN SCHEMA).

Currently, TABLES IN SCHEMA publishes all tables in a schema with no
way to exclude a subset. Users who want to skip a few tables must
switch to an explicit FOR TABLE list, which loses the convenience of
schema-level publishing and requires ongoing maintenance as tables are
added.

Proposed syntax:
------------------------
CREATE PUBLICATION pub FOR TABLES IN SCHEMA s1 EXCEPT (s1.t1, s1.t2);
ALTER PUBLICATION pub ADD TABLES IN SCHEMA s1 EXCEPT (s1.t1);
ALTER PUBLICATION pub SET TABLES IN SCHEMA s1 EXCEPT (s1.t1);

Note: Tables in the EXCEPT clause must be schema-qualified to avoid
ambiguity and must belong to the published schema; otherwise, an error
is raised.

Rules and behavior:
----------------------------
1) TABLES IN SCHEMA can be combined with FOR TABLE, but EXCEPT applies
only to the schema clause and must appear immediately after it.

Supported:
  CREATE PUBLICATION pub FOR TABLES IN SCHEMA s1 EXCEPT (s1.t1), TABLE s2.t1;
  CREATE PUBLICATION pub FOR TABLE s2.t1, TABLES IN SCHEMA s1 EXCEPT (s1.t1);
  CREATE PUBLICATION pub FOR TABLES IN SCHEMA s1 EXCEPT (s1.t1), s2
EXCEPT (s2.t1)

Not supported:
  CREATE PUBLICATION pub FOR TABLES IN SCHEMA s1,  TABLE s2.t1 EXCEPT (s1.t1);

-- This same rule applies to ALTER PUBLICATION ... ADD/SET.

2) Conflicting definitions
Specifying the same table both in the EXCEPT clause and explicitly in
the TABLE clause results in an error, as this creates a conflicting
definition for the publication.

3) "ALTER PUBLICATION ... DROP TABLES IN SCHEMA" does not support
EXCEPT clause. Whereas, dropping a schema also removes any associated
entries from the EXCEPT list of the publication.
 -- To only remove/update except list entries, use SET instead.

4) Consistency with ALL TABLES EXCEPT rules:
  4a) Excluding a partitioned root excludes all its partitions
  4b) Individual partitions cannot be excluded directly; exclude the root table.
  4c) Excluding an inheritance parent (without ONLY) also excludes its children.

The patches are divided into three parts to simplify review:
  Patch-001: Basic framework to support EXCEPT in CREATE PUBLICATION
... TABLES IN SCHEMA
  Patch-002: Extend support to ALTER PUBLICATION ... ADD TABLES IN SCHEMA
  Patch-003: Extend support to ALTER PUBLICATION ... SET TABLES IN SCHEMA

The patches are attached, feedback and suggestions are welcome.

[1] https://www.postgresql.org/message-id/CALDaNm3%3DJrucjhiiwsYQw5-PGtBHFONa6F7hhWCXMsGvh%3DtamA%40mail.gmail.com

--
Thanks,
Nisha