Support setting numShards > 1 in SMB transforms

[clairemcginty]

Number of output buckets in SMB transform can be configured using TargetParallelism:

sc.sortMergeTransform(
      classOf[String],
      read,
      TargetParallelism.of(1024) // Writes 1024 output buckets
    )
   .to(...)
   .via(...)

But in the case of skewed data, there's currently no way to set number of output shards--it's always hardcoded to 1.

We'd have to do some thinking on how to implement this -- since all shards would be written from the same worker core, we'd have to implement a round-robin type algorithm to assign shards, and maintain numShards open file handles while writing.