Add support for batching in the generator framework

[nickmoorman]

In some cases, it may be desired to use a generator but handle the output in batches instead of individually. For example, maybe you want to list the files in a Google Cloud Storage bucket and then create BigQuery load jobs with them in batches of 10 (instead of a single load job per file). It should be possible to specify batching parameters in a generator config to achieve this result.