Add Topic and Partition ID to Protocol.Fetch.Message

[yogodoshi]

On the Message struct there is the offset field but not the topic or the partition ID. It would be very useful for me if them were there as well, here is why:

topic_name
|> retrieve_partitions_ids
|> create_streams(topic_name)
|> Flow.from_enumerables()
|> Flow.map(&prepare_for_partition/1)
|> Flow.partition(key: {:elem, 0}, window: periodic_window)
|> Flow.reduce(&accumulator_tuple/0, &save_kafka_message_to_ets/2)
(more Flow stuff)
|> Flow.on_trigger(&send_to_redshift/3)
|> Flow.run()

I have a Flow with a KafkaEx.stream for each partition and down the road, I don't really have a way to know from which partition each message is from, which prevents me from committing the offset.

I would think that if this information would be great to have in my scenario, it would probably also be useful to others as a Map-Reduce is probably frequently used with KafkaEx.stream 🤔

If you think this change makes sense let me know and I can try opening a PR implementing it :)

[joshuawscott]

Fixed in #360