Provide a way to merge Kll sketches deterministically

[thomasrebele]

As explained in https://github.com/apache/datasketches-java/issues/693, it would be very helpful for certain downstream projects to be able to use the KLL sketches, while still getting a deterministic result.

This PR proposes to add a method KllSketch#merge(KllSketch, Random) as an alternative. Existing code will continue to run as before. New callers can consider to use the newly added method if they need deterministic results. A warning has been added to the javadoc of the new method, to make it clear that the error guarantees of the KLL sketch algorithm might no longer be valid.

[jmalkin]

I am not in favor of this behavior. It is nearly always the wrong thing for users to do.