CDK-496: Added the type parameter to the Datasets API

I also went through and extended the use of the type parameter down to at least Avro files
and CSV files. In order to support Parquet and HBase, I think more work needs to be done
on the underlying stores. But if I missed something, I can get those in before the merge.
Will do. I typically try to avoid them, but my IDE recently started putting them in for me without me noticing. I think I've fixed the config, so it should be easier to avoid in the future.