Enable injecting custom partition assignor

[vertexclique]

This one is little bit tricky. You need to inject custom partition assignor as raw pointer allocated on heap by using the implementaion of this trait: https://docs.rs/rdkafka/latest/rdkafka/consumer/trait.ConsumerContext.html need to make it inside a object allocated on heap and injected into here: https://github.com/vertexclique/callysto/blob/625f38587d3c0695c116b8c3662c74bbb2db7704/src/kafka/enums.rs#L56

HOW

How I will make it inside an Box<dyn ConsumerContext>? What I need to know? What is a Box? Why Box? What does heap allocated object means? What is dyn? @ansrivas can help you on this.