Combine operation should support only converting to accumulators

For streaming pipelines, we want to be able to lift the combiner into the MergeBuckets without having to also do a PartialGroupByKey before the shuffle. We don't want to do the PGBK since it could cause non-deterministic results when used with some triggers.

We propose adding a new URN for doing just the convert to accumulators step and adding support for it in Java/Python/Go.