Support column list for foreign key ON DELETE SET actions on PostgreSQL

Support column list for foreign key ON DELETE SET actions on PostgreSQL

Added support for specifying a list of columns for ON DELETE SET
(NULL|DEFAULT) actions of foreign key definition on PostgreSQL. This is
handled on both compiler and reflection sides.

In order to make it possible to override the logic of
DDLCompiler.define_constraint_cascades() in derived classes, namely
PGDDLCompiler here, we add two methods,
define_constraint_ondelete_cascade() and
define_constraint_onupdate_cascade(), the former being overridden in
PGDDLCompiler.

Test cases (tables definition) are taken from PostgreSQL test suite.

Fixes: #11595
Fixes: #11946