support create/drop events for all CREATE/DROP

support create/drop events for all CREATE/DROP

Implemented the DDL event hooks :meth:`.DDLEvents.before_create`,
:meth:`.DDLEvents.after_create`, :meth:`.DDLEvents.before_drop`,
:meth:`.DDLEvents.after_drop` for all :class:`.SchemaItem` objects that
include a distinct CREATE or DROP step, when that step is invoked as a
distinct SQL statement, including for :class:`.ForeignKeyConstraint`,
:class:`.Sequence`, :class:`.Index`, and PostgreSQL's
:class:`_postgresql.ENUM`.

Fixes: #8394
Change-Id: Iee6274984e794f50103451a04d089641d6ac443a