Ensure custom ops have consistent typing behavior, boolean support

Ensure custom ops have consistent typing behavior, boolean support

Refined the behavior of :meth:`.Operators.op` such that in all cases,
if the :paramref:`.Operators.op.is_comparison` flag is set to True,
the return type of the resulting expression will be
:class:`.Boolean`, and if the flag is False, the return type of the
resulting expression will be the same type as that of the left-hand
expression, which is the typical default behavior of other operators.
Also added a new parameter :paramref:`.Operators.op.return_type` as well
as a helper method :meth:`.Operators.bool_op`.

Change-Id: Ifc8553cd4037d741b84b70a9702cbd530f1a9de0
Fixes: #4063