Add a multi-processing worker that runs both gRPC and kafka consumer

[markstory]

• Get both a gRPC interface and arroyo consumer running in the same process.
• Can use threading or multiprocessing to make this happen.

Rationale

Having the ability to run both the consumer + grpc service within a single process will help us simplify production deployments as we won't need to use a shared volume between two containers within the consumer pod.