Kafka Streams runner: splittable DoFn

The Kafka Streams runner does not support splittable DoFn, bounded or unbounded. Sources are read through the deprecated primitive Read, which the runner forces Read-based SDFs into during translation.

Supporting SDF means handling restrictions, checkpointing and residuals over the Fn API, along with dynamic splitting. The UsesBoundedSplittableParDo and UsesUnboundedSplittableParDo categories are excluded from the ValidatesRunner suite.