Configuration : Allow override for external consumer/producer

[LGouellec]

Description

For now, you can't override the configuration for external consumer and producer in the external stream thread. The aim of this feature is to purpose a new prefix configuration to override the fetch.min.bytes just for the external consumer for instance.

var config = new StreamConfig();
config.Add("external.consumer.fetch.min.bytes", 1000);
config.Add("external.producer.acks", Acks.Leader);