PG: Support creation and usage of custom types

[matthias-Q]

Postgres allows to create custom types. The creation of types, tables that are using these types and the selection of columns with these types is not yet supported:

Here is an example:

CREATE TYPE obj AS (
    elem1 TEXT,
    elem2 INT
);

CREATE TABLE obj_tab (
    object obj
);

SELECT
    (object).elem1,
    (object).elem2,
FROM obj_tab

Docs: https://www.postgresql.org/docs/current/sql-createtype.html