Create new event for collection add w/o mutation

Create new event for collection add w/o mutation

Fixed issue when using :paramref:`_orm.relationship.cascade_backrefs`
parameter set to ``False``, which per :ref:`change_5150` is set to become
the standard behavior in SQLAlchemy 2.0, where adding the item to a
collection that uniquifies, such as ``set`` or ``dict`` would fail to fire
a cascade event if the object were already associated in that collection
via the backref. This fix represents a fundamental change in the collection
mechanics by introducing a new event state which can fire off for a
collection mutation even if there is no net change on the collection; the
action is now suited using a new event hook
:meth:`_orm.AttributeEvents.append_wo_mutation`.

Fixes: #6471
Change-Id: Ic50413f7e62440dad33ab84838098ea62ff4e815