reflect PG custom types besides ENUM / support composite types

[Mike Bayer]

this would expand upon _load_enums() in postgresql/base.py, so that any kind of custom type is recognized during reflection. We'd likely have to create some kind of "ad-hoc" type that is declared like this:

from sqlalchemy.dialects.postgresql import TYPE
Column("mycolumn", TYPE("MY_CUSTOM_TYPE"))

[Mike Bayer]

Issue #3555 was marked as a duplicate of this issue.

[Mike Bayer]

see also #3566