Simplify load balancing configuration

[aikoven]

Currently to enable round-robin load balancing one must create a channel as follows:

const channel = createChannel(address, undefined, {
  'grpc.service_config': JSON.stringify({
    loadBalancingConfig: {round_robin: {}},
  }),
});

We should at least have this documented somewhere.

We could also support typing the grpc.service_config option value as object.