Add a modular transformation capability to multicast client

Current multicast client mirrors the bytes inbound to the destination topic. In cases where the inbound data in binary and clients are JSON (for example), provide a shared module capability where end customers can plug UDF-based transformations (a la Dataflow templates) authored in Go (or even another language - BQ's is JS) that can be deployed on premise such that the data is already transformed when it egresses the customer's perimeter.