Create object- and column-oriented versions of AssociationProxyInstance

Create object- and column-oriented versions of AssociationProxyInstance

The :class:`.AssociationProxy` now has standard column comparison operations
such as :meth:`.ColumnOperators.like` and
:meth:`.ColumnOperators.startswith` available when the target attribute is a
plain column - the EXISTS expression that joins to the target table is
rendered as usual, but the column expression is then use within the WHERE
criteria of the EXISTS.  Note that this alters the behavior of the
``.contains()`` method on the association proxy to make use of
:meth:`.ColumnOperators.contains` when used on a column-based attribute.

Fixes: #4351
Change-Id: I310941f4e8f778c200f8144a26a89e5364cd4dfb