REST perf2perf transform

[mvahowe]

The idea here is to design a transform that POSTs its sources to a specified REST endpoint and unpacks the HTTP response as its outputs. This, combined with, say, a Python Flask server, would enable non-JS processing to be included in the pipeline.

The issue says perf2perf because it would currently be hard to provide flexible input/output options for a single transform. We could produce other REST transform templates for other cases.