Improve identity column reflection

Improve identity column reflection

Add SQL typing to reflection query used to retrieve a the structure
of IDENTITY columns, adding explicit JSON typing to the query to suit
unusual PostgreSQL driver configurations that don't support JSON natively.

Fixed issue affecting PostgreSQL 17.3 and greater where reflection of
domains with "NOT NULL" as part of their definition would include an
invalid constraint entry in the data returned by
:meth:`_postgresql.PGInspector.get_domains` corresponding to an additional
"NOT NULL" constraint that isn't a CHECK constraint; the existing
``"nullable"`` entry in the dictionary already indicates if the domain
includes a "not null" constraint.   Note that such domains also cannot be
reflected on PostgreSQL 17.0 through 17.2 due to a bug on the PostgreSQL
side; if encountering errors in reflection of domains which include NOT
NULL, upgrade to PostgreSQL server 17.3 or greater.

Fixes: #11751
Change-Id: I8e69de51601dca3257186e38c6f699fbfd9014c6