Add support for altering CHECK constraint enforceability

Add support for altering CHECK constraint enforceability

This commit adds support for ALTER TABLE ALTER CONSTRAINT ... [NOT]
ENFORCED for CHECK constraints.  Previously, only foreign key
constraints could have their enforceability altered.

When changing from NOT ENFORCED to ENFORCED, the operation not only
updates catalog information but also performs a full table scan in
Phase 3 to validate that existing data satisfies the constraint.

For partitioned tables and inheritance hierarchies, the operation
recurses to all child tables.  When changing to NOT ENFORCED, we must
recurse even if the parent is already NOT ENFORCED, since child
constraints may still be ENFORCED.

Author: Jian He <jian.universality@gmail.com>
Reviewed-by: Robert Treat <rob@xzilla.net>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Amul Sul <sulamul@gmail.com>
Reviewed-by: Zsolt Parragi <zsolt.parragi@cybertec.at>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/CACJufxHCh_FU-FsEwsCvg9mN6-5tzR6H9ntn+0KUgTCaerDOmg@mail.gmail.com