Add partitioner type projection method

To get CDK-342 done, we updated the identity partitioner to use Object.class as the type it produces. That's not entirely true, but was necessary to separate the partition strategy definition from the schema. Because partitioners (and strategies) are Immutable, we can't set this later when the schema is known. I think the solution is to add a partitioner method that gives the output type when given a particular input type:

FieldPartitioner<S, T>:
  T apply(S)
  Class<? extends T> getType(Class<? extends S>)

This may not be needed, but I think it's the right way to get the type.