Relax "ambiguous" association proxy restrictions, support Proxy

Relax "ambiguous" association proxy restrictions, support Proxy

Fixed issue in association proxy due to :ticket:`3423` which caused the use
of custom :class:`.PropComparator` objects with hybrid attribites, such as
the one demonstrated in  the ``dictlike-polymorphic`` example to not
function within an association proxy.  The strictness that was added in
:ticket:`3423` has been relaxed, and additional logic to accomodate for
an association proxy that links to a custom hybrid have been added.

Fixes: #4446
Change-Id: I8addc80f51094769915ac2dce1a301bd72ee7433