ConstraintStreams: accumulate or outer right join

This pattern is common:

   f.forEach(MyEntity)
    .groupBy(MyEntity::myGrouping, count())
    .join(MinMaxRule, ... same myGrouping ...)

It's also buggy. If nothing is assigned to the grouping, the MinMaxRule isn't checked.

This is because join() is an inner join in SQL talk. We need an outer right join instead.
Some models have helper code for that:

    @SafeVarargs
    private <A, B> BiConstraintStream<A, B> joinOuterRight(UniConstraintStream<A> sourceStream,
            Class<B> joinedClass, BiJoiner<A, B>... joiners) {
        return sourceStream.join(joinedClass, joiners)
                .concat(sourceStream.ifNotExists(joinedClass, joiners));
    }

Let's standardize it.

Proposal A)

Add syntactic sugar + tests for Uni/Bi/Tri/QuadConstraintStream.joinOuterRight(...), as shown above.
Later implementations can potentially optimize the implementation, but for now the facade is enough.

Naming to be discussed.
Later we might want to implement a left outer join (.joinOuterLeft()) and a full outer join (.joinOuterFull()) too.

Start with the UniConstraintStream, BiConstraintStream, TriConstraintStream, and QuadConstraintStream join and ifNotExists entry points, using the helper pattern shown in the issue as the reference. Add tests for the proposed outer-right-join behavior across all four stream types, and confirm that the naming and behavior are consistent before considering the work done.