Add a modular transformation capability to multicast client

[salsferrazza]

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.